Tradefox Tools

HVAC error codes. Decode the fault. Know when to stop.

Find what an air conditioner, heat pump or boiler fault code means, what you can safely check, and when to call an engineer.

Safety first. Isolate the power before removing filters or covers. In the UK only F-gas certified engineers may work on refrigerant circuits, and only Gas Safe registered engineers may work on gas appliances. If you smell gas, call the National Gas Emergency line on 0800 111 999.

Type a code (e.g. U4, E18, F.22) or choose a brand to browse.

Meanings and checks are written by Tradefox as a guide. Fault codes are the manufacturers'; trademarks belong to their owners. This tool doesn't replace the manufacturer's manual or a qualified engineer.

Every HVAC and heating error code, by brand

All 560 codes in this look-up from 20 manufacturers, grouped by brand and model series. Open a series to read what each code means, who should deal with it and what to check first.

Daikin Air conditioning & heat pumps

Daikin Split / wall units (Perfera FTXM, Emura FTXJ, Comfora FTXP) 30 codes

Air conditioner · Covers FTXM-R Perfera, FTXJ-A Emura 3, FTXP-M Comfora, FTXF Sensira, ATXF, RXM-R outdoor, RXJ-A outdoor, 2MXM/3MXM/4MXM multi

Indoor unit circuit board has detected an internal fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Condensate level too high in the indoor unit (drain system fault).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Indoor coil protection: freeze-up in cooling or very high pressure in heating.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Indoor fan motor is not running or not reporting its speed.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Air purifier / streamer unit fault (where fitted).

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Indoor heat-exchanger thermistor reading is out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Front panel / louvre drive fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Indoor room-air thermistor reading is out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor unit circuit board fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Compressor overload protection has tripped.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Compressor failed to start or is locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Outdoor fan motor locked or not reaching speed.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Input current too high for the operating condition.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Four-way (reversing) valve not switching between heating and cooling correctly.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor discharge pipe temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

High pressure protection (cooling) or heat exchanger overheating.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Compressor drive sensor system fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Compressor rotor position could not be detected at start.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Outdoor current sensor (CT) fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Outdoor air thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Discharge pipe thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor heat-exchanger thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor electrical box temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter heat-sink (radiation fin) temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter output overcurrent detected.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Heat-sink thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Low refrigerant / gas shortage detected from temperature readings.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Supply voltage too low/high or DC voltage abnormal.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Communication failure between indoor and outdoor units.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Indoor and outdoor units are not a compatible combination.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Daikin's standard malfunction code set; shown on remote via the check/cancel button.

Source: Daikin UK installer support / My Daikin document library (opens in new tab).

Daikin Sky Air (FBA, FCAG, FUA, FHA) indoor + RZAG/RZASG outdoor 30 codes

Air conditioner · Covers FBA-A ducted, FCAG-B cassette, FUA-A ceiling suspended, FHA-A ceiling, RZAG-A outdoor, RZASG outdoor

Indoor unit circuit board has detected an internal fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Condensate level too high in the indoor unit (drain system fault).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Indoor coil protection: freeze-up in cooling or very high pressure in heating.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Indoor fan motor is not running or not reporting its speed.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Air purifier / streamer unit fault (where fitted).

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Indoor heat-exchanger thermistor reading is out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Front panel / louvre drive fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Indoor room-air thermistor reading is out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor unit circuit board fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Compressor overload protection has tripped.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Compressor failed to start or is locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Outdoor fan motor locked or not reaching speed.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Input current too high for the operating condition.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Four-way (reversing) valve not switching between heating and cooling correctly.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor discharge pipe temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

High pressure protection (cooling) or heat exchanger overheating.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Compressor drive sensor system fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Compressor rotor position could not be detected at start.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Outdoor current sensor (CT) fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Outdoor air thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Discharge pipe thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor heat-exchanger thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor electrical box temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter heat-sink (radiation fin) temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter output overcurrent detected.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Heat-sink thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Low refrigerant / gas shortage detected from temperature readings.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Supply voltage too low/high or DC voltage abnormal.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Communication failure between indoor and outdoor units.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Indoor and outdoor units are not a compatible combination.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Same malfunction code family as split units; read from BRC wired controller.

Source: Daikin UK installer support / My Daikin document library (opens in new tab).

Daikin Altherma 3 R / H air-to-water heat pump 11 codes

Heat pump · Covers Altherma 3 R (EHBH/EHVH + ERGA), Altherma 3 H HT (EPRA), Altherma 3 M monobloc (EBLA/EDLA)

Water flow too low through the heat pump.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Return-water thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Leaving-water thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Plate heat exchanger freeze protection has operated.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Backup heater thermal protector has opened.

Competent engineer · confirm in your model's manual

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

Hydro-box PCB / memory fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Refrigerant liquid thermistor reading out of range.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor fan motor fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Communication fault between hydro-box and outdoor unit.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Possible refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Incompatible unit combination / wrong setting.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Source: Daikin UK installer support / My Daikin document library (opens in new tab).

Mitsubishi Electric Air conditioning & heat pumps

Mitsubishi Electric M-Series wall units (MSZ-LN, MSZ-AP, MSZ-HR) 26 codes

Air conditioner · Covers MSZ-LN Zen, MSZ-AP, MSZ-HR, MSZ-EF, MUZ-LN/AP outdoor, MXZ multi outdoor

E0/E3

Remote controller transmission fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E1/E2

Remote controller board / display fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E4/E5

Remote controller signal not being received.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Indoor-outdoor communication error (as seen by the indoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E8/E9

Indoor-outdoor communication error (as seen by the outdoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

EA/Eb

Indoor-outdoor wiring miswired, or too many indoor units connected.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Room/return-air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor liquid-pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Drain float switch connector open / drain sensor fault.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Drain overflow protection (condensate pump / drain blocked).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Indoor coil freeze or overheat protection.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Pipe temperature abnormal - unit not seeing the expected temperature change.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Indoor two-phase pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor control board fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

High pressure protection (high-pressure switch operated).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Discharge temperature too high or refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor discharge thermistor open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor thermistor (pipe/ambient/fin) open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Inverter heat-sink temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Compressor overcurrent / power module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Discharge superheat too low.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor fan motor stopped abnormally.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Supply voltage abnormal (over/under voltage, phase loss).

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Compressor overcurrent - compressor locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Low pressure protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor overcurrent interruption.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Some M-Series units show faults as indoor LED blink patterns instead; the check code (via remote CHECK) is listed here.

Source: Mitsubishi Electric UK Document Library (opens in new tab).

Mitsubishi Electric Mr Slim (PEAD/PKA/PLA indoor, PUZ/SUZ outdoor) 26 codes

Air conditioner · Covers PEAD-M ducted, PKA-M wall, PCA-M ceiling, PUZ-ZM Power Inverter, SUZ-M outdoor

E0/E3

Remote controller transmission fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E1/E2

Remote controller board / display fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E4/E5

Remote controller signal not being received.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Indoor-outdoor communication error (as seen by the indoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E8/E9

Indoor-outdoor communication error (as seen by the outdoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

EA/Eb

Indoor-outdoor wiring miswired, or too many indoor units connected.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Room/return-air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor liquid-pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Drain float switch connector open / drain sensor fault.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Drain overflow protection (condensate pump / drain blocked).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Indoor coil freeze or overheat protection.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Pipe temperature abnormal - unit not seeing the expected temperature change.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Indoor two-phase pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor control board fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

High pressure protection (high-pressure switch operated).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Discharge temperature too high or refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor discharge thermistor open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor thermistor (pipe/ambient/fin) open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Inverter heat-sink temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Compressor overcurrent / power module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Discharge superheat too low.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor fan motor stopped abnormally.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Supply voltage abnormal (over/under voltage, phase loss).

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Compressor overcurrent - compressor locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Low pressure protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor overcurrent interruption.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Source: Mitsubishi Electric UK Document Library (opens in new tab).

Mitsubishi Electric SLZ / PLA ceiling cassettes 26 codes

Air conditioner · Covers SLZ-M 4-way cassette, PLA-M cassette

E0/E3

Remote controller transmission fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E1/E2

Remote controller board / display fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E4/E5

Remote controller signal not being received.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Indoor-outdoor communication error (as seen by the indoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E8/E9

Indoor-outdoor communication error (as seen by the outdoor unit).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

EA/Eb

Indoor-outdoor wiring miswired, or too many indoor units connected.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Room/return-air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor liquid-pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Drain float switch connector open / drain sensor fault.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Drain overflow protection (condensate pump / drain blocked).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Indoor coil freeze or overheat protection.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Pipe temperature abnormal - unit not seeing the expected temperature change.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Indoor two-phase pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor control board fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

High pressure protection (high-pressure switch operated).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Discharge temperature too high or refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor discharge thermistor open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor thermistor (pipe/ambient/fin) open/short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Inverter heat-sink temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Compressor overcurrent / power module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Discharge superheat too low.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor fan motor stopped abnormally.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Supply voltage abnormal (over/under voltage, phase loss).

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Compressor overcurrent - compressor locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Low pressure protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor overcurrent interruption.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Source: Mitsubishi Electric UK Document Library (opens in new tab).

Mitsubishi Electric Ecodan air source heat pump (PUZ-WM / PUHZ-W with FTC controller) 16 codes

Heat pump · Covers PUZ-WM50/60/85/112 Ecodan, PUHZ-W Ecodan (older), EHST20D / ERST20D FTC6 cylinder, EHSD hydrobox

Primary circuit water temperature too high.

Competent engineer · confirm in your model's manual

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

Cylinder (DHW) water temperature too high.

Competent engineer · confirm in your model's manual

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

Water-side thermistor fault (flow/return/tank).

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Primary circuit freeze protection has operated.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Heating not reaching temperature as expected.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Low primary water flow detected by the flow switch/sensor.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Boiler (hybrid) flow temperature too high.

Competent engineer · confirm in your model's manual

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

Boiler (hybrid) temperature thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Boiler (hybrid) operation fault.

Gas Safe registered engineer · confirm in your model's manual

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Communication fault between FTC controller and outdoor unit.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

High pressure protection.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Discharge temperature too high / refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Compressor overcurrent / power module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Outdoor fan motor stopped.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Compressor overcurrent interruption.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Hydrobox/FTC 'L' codes plus outdoor 'U' codes.

Source: Mitsubishi Electric UK Document Library (opens in new tab).

Fujitsu (GENERAL) Air conditioning & heat pumps

Fujitsu (GENERAL) Airstage / wall split (ASYG, ASYA, ARYG duct, AUYG cassette) 24 codes

Air conditioner · Covers ASYG-KMCC/KMTA, ASYG-KLCA, ARYG ducted, AUYG cassette, AOYG outdoor

Serial communication fault between indoor and outdoor units.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Wired remote controller communication fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Automatic pipe check / airflow adjustment did not complete.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Indoor unit PCB fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Indoor room temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor heat-exchanger temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor fan motor fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Drain pump / condensate fault.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Outdoor main PCB fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Inverter circuit fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

PFC circuit (power factor correction) fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Discharge temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor heat-exchanger temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor air temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Heat-sink temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Current sensor fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Pressure sensor / switch fault.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Trip (overcurrent) detection.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Compressor rotor position / motor control fault.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Outdoor fan motor fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Four-way valve fault.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Discharge temperature protection operated.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor temperature protection operated.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Low pressure protection operated.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Error number appears on wired controller; IR models flash OPERATION/TIMER LEDs.

Source: GENERAL (Fujitsu General) UK downloads - installation & operation manuals (opens in new tab).

LG Air conditioning & heat pumps

LG Split AC (Artcool, Standard Plus, Dualcool) 25 codes

Air conditioner · Covers Artcool (AC09BK), Standard Plus (PC09SQ), Dualcool (S12ET), MU2R/MU3R multi outdoor

CH01

Indoor room-air sensor open or short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH02

Indoor pipe (inlet) sensor open or short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH05

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

CH06

Indoor pipe (outlet) sensor open or short.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH09

Indoor EEPROM / option data fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

CH10

Indoor fan motor locked or not reporting speed.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

CH21

Inverter power module (IPM) fault / DC peak current.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

CH22

Input current too high (overcurrent).

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

CH23

DC link voltage too low or too high.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

CH26

Compressor start / position detection fault.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

CH27

PFC (power factor correction) fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

CH29

Compressor phase overcurrent.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

CH32

Discharge pipe temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

CH34

High pressure protection.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

CH35

Low pressure protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

CH41

Discharge pipe sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH44

Outdoor air sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH45

Outdoor coil (mid) sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH46

Suction pipe sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH53

Communication fault reported by the outdoor unit.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

CH60

Outdoor EEPROM fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

CH61

Outdoor coil temperature too high (cooling).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

CH62

Heat-sink temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

CH65

Heat-sink sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH67

Outdoor fan locked.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Shown as CH followed by a number on the display/remote.

Source: LG UK Manuals & Software (opens in new tab).

LG Therma V air-to-water heat pump 9 codes

Heat pump · Covers Therma V R32 monobloc (HM091MR/HM161MR), Therma V split (HN/HU)

CH03

Wired remote controller communication fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

CH05

Communication fault between hydro board and outdoor/inverter board.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

CH14

Water flow switch / flow rate fault - not enough water flow.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

CH16

Water-side temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

CH21

Inverter power module fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

CH32

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

CH34

High pressure protection.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

CH35

Low pressure protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

CH67

Outdoor fan locked.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Source: LG UK Manuals & Software (opens in new tab).

Samsung Air conditioning & heat pumps

Samsung WindFree / wall split 15 codes

Air conditioner · Covers WindFree Elite, WindFree Comfort, WindFree Avant, AR35 / AR70 wall units

E101

Communication fault between indoor and outdoor units (indoor side).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E121

Indoor room temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E122

Indoor evaporator inlet sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E123

Indoor evaporator outlet sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E154

Indoor fan motor fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

E201

Indoor-outdoor communication fault (wiring or address).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E202

Indoor-outdoor communication fault (no response).

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E221

Outdoor air temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E231

Outdoor coil (condenser) temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E251

Discharge temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E416

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

E458

Outdoor fan motor fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

E461

Compressor failed to start.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

E464

Inverter IPM overcurrent.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

E554

Refrigerant leak / gas shortage detected.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Source: Samsung UK user manuals & guides (opens in new tab).

Samsung EHS Mono / Split air-to-water heat pump 7 codes

Heat pump · Covers EHS Mono R290 (AE-GXE), EHS Mono HT Quiet, EHS Split (AE-RNE/RXE)

E101

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E201

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E221

Outdoor air sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E416

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

E464

Inverter IPM overcurrent.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

E554

Refrigerant shortage detected.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

E911

Water flow too low (flow sensor / flow switch).

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Source: Samsung UK user manuals & guides (opens in new tab).

Panasonic Air conditioning & heat pumps

Panasonic Etherea / TZ / wall split 37 codes

Air conditioner · Covers Etherea Z (CS-Z), TZ (CS-TZ), Flagship (CS-XZ), CU-Z / CU-TZ outdoor

H11

Indoor-outdoor communication fault.

Competent engineer · checked against manufacturer source

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

H12

Indoor and outdoor capacities don't match.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

H14

Indoor intake-air sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H15

Compressor temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H16

Outdoor current transformer (CT) fault.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

H19

Indoor fan motor locked.

Competent engineer · checked against manufacturer source

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

H21

Indoor float switch has operated (drain).

Safe to check yourself · checked against manufacturer source

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

H23

Indoor heat-exchanger sensor 1 fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H24

Indoor heat-exchanger sensor 2 fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H25

Indoor ion device fault.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

H27

Outdoor air sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H28

Outdoor heat-exchanger sensor 1 fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H30

Outdoor discharge pipe sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H32

Outdoor heat-exchanger sensor 2 fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H33

Indoor/outdoor wiring or voltage mismatch.

Competent engineer · checked against manufacturer source

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

H34

Outdoor heat-sink sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H36

Outdoor gas pipe sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H37

Outdoor liquid pipe sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H38

Indoor and outdoor brand codes don't match.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

H41

Wiring or piping connected to the wrong unit/port.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

H64

Outdoor high-pressure sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H67

nanoe device fault.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

H97

Outdoor fan motor locked.

Safe to check yourself · checked against manufacturer source

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

H98

Indoor high-pressure protection (heating).

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

H99

Indoor coil freeze protection.

Safe to check yourself · checked against manufacturer source

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

F11

Four-way valve switching fault.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F16

Total running current protection.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F87

Control box overheating protection.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F90

PFC circuit protection / compressor communication fault.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F91

Refrigeration cycle abnormality (often low charge).

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F93

Compressor speed abnormal.

F-gas certified engineer · checked against manufacturer source

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

F94

Compressor discharge pressure protection.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F95

Outdoor high pressure protection (cooling).

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F96

Power transistor (IPM) overheating.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F97

Compressor overheating.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F98

Total running current protection.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F99

Outdoor DC peak current detected.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Read via the remote's self-diagnosis (hold CHECK).

Source: Panasonic - Air conditioner error codes (self-diagnosis function) (opens in new tab).

Panasonic Aquarea air-to-water heat pump 17 codes

Heat pump · Covers Aquarea J/K/L generation monobloc, Aquarea split (WH-SDC/WH-UD), Aquarea All-in-One (WH-ADC)

H15

Compressor temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H62

Water flow switch fault - flow too low.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

H70

Backup heater overload protection operated.

Competent engineer · confirm in your model's manual

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

H72

Tank temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H76

Indoor unit - control panel communication fault.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

H90

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

H99

Indoor heat-exchanger freeze protection.

Safe to check yourself · confirm in your model's manual

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F12

Pressure switch activated.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F15

Outdoor fan motor locked.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

F20

Compressor overheating protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F22

Power transistor (IPM) overheating.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F27

Pressure switch fault.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F36

Outdoor air sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F37

Water inlet sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F40

Discharge sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F45

Water outlet sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F95

High pressure protection (cooling).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Shares H/F code scheme with Panasonic AC; water-side codes not line-checked.

Source: Panasonic - Air conditioner error codes (self-diagnosis function) (opens in new tab).

Toshiba Air conditioning & heat pumps

Toshiba RAS split / Digital Inverter / Seiya / Haori 28 codes

Air conditioner · Covers Seiya (RAS-B10E2KVG), Haori (RAS-B10N4KVRG), Shorai Edge, Daiseikai, Digital Inverter RAV ducted/cassette

E01

No signal from the wired remote controller.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E03

Indoor unit not receiving signals from the controller/central control.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E04

Indoor-outdoor serial communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E08

Duplicate indoor unit address.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

E09

Two master remote controllers set.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

E10

Indoor PCB internal communication fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

F01

Indoor heat-exchanger sensor (TCJ) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F02

Indoor heat-exchanger sensor (TC) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F04

Discharge temperature sensor (TD) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F06

Outdoor heat-exchanger sensor (TE) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F07

Outdoor liquid sensor (TL) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F08

Outdoor air sensor (TO) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F10

Indoor room/return air sensor (TA) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F12

Suction temperature sensor (TS) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F13

Heat-sink sensor (TH) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H01

Compressor breakdown / overcurrent at start.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

H02

Compressor locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

H03

Current detection circuit fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P03

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P04

High pressure switch operated.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

P07

Heat-sink overheat.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P10

Indoor drain overflow / float switch operated.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

P12

Indoor fan motor fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

P15

Gas leak (refrigerant shortage) detected.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P19

Four-way valve fault.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P22

Outdoor fan fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

P26

Inverter IGBT short-circuit protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P29

Compressor position detection fault.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Source: Toshiba Air Conditioning UK - manuals & downloads (opens in new tab).

Toshiba SMMS / VRF indoor units (TCC-Link) 28 codes

Air conditioner · Covers SMMS-u, SHRM, MMY outdoor

E01

No signal from the wired remote controller.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E03

Indoor unit not receiving signals from the controller/central control.

Competent engineer · confirm in your model's manual

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E04

Indoor-outdoor serial communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E08

Duplicate indoor unit address.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

E09

Two master remote controllers set.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

E10

Indoor PCB internal communication fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

F01

Indoor heat-exchanger sensor (TCJ) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F02

Indoor heat-exchanger sensor (TC) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F04

Discharge temperature sensor (TD) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F06

Outdoor heat-exchanger sensor (TE) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F07

Outdoor liquid sensor (TL) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F08

Outdoor air sensor (TO) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F10

Indoor room/return air sensor (TA) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F12

Suction temperature sensor (TS) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F13

Heat-sink sensor (TH) fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

H01

Compressor breakdown / overcurrent at start.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

H02

Compressor locked.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

H03

Current detection circuit fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P03

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P04

High pressure switch operated.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

P07

Heat-sink overheat.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P10

Indoor drain overflow / float switch operated.

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

P12

Indoor fan motor fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

P15

Gas leak (refrigerant shortage) detected.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P19

Four-way valve fault.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

P22

Outdoor fan fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

P26

Inverter IGBT short-circuit protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

P29

Compressor position detection fault.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Source: Toshiba Air Conditioning UK - manuals & downloads (opens in new tab).

Hitachi Air conditioning & heat pumps

Hitachi Utopia / RAS split & ducted 23 codes

Air conditioner · Covers Utopia RAS-H(V)RNM, RAK wall split, RCI cassette

Indoor protection device operated (often drain float switch).

Safe to check yourself · confirm in your model's manual

Isolate power. Check whether the condensate drain or tray is blocked or the pipe is kinked or back-falling.
Clear the drain line / clean the tray if accessible; check the condensate pump (if fitted) and its float switch.
If water keeps backing up, have an engineer check the pump, float switch and pipe gradient.

Outdoor protection device operated (often high pressure).

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Indoor-outdoor transmission fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Inverter - outdoor PCB transmission fault.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Power supply phase fault.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Voltage drop / abnormal supply voltage.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Discharge superheat too low.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor top / discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Indoor inlet-air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor outlet-air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor freeze-protection (liquid pipe) thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor gas-pipe thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor fan motor protection operated.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Compressor discharge thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor air thermistor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Incorrect capacity setting / combination.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Incorrect indoor unit number setting.

Competent engineer · confirm in your model's manual

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

Overload protection in cooling.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Overload protection in heating.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Low pressure protection (suction pressure too low).

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Inverter overcurrent protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Inverter fin temperature too high.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Source: Hitachi Cooling & Heating (UK site) (opens in new tab).

Carrier Furnaces & HVAC (US)

Carrier 355MAV / Infinity two-stage condensing gas furnace 18 codes

Gas furnace · Covers 355MAV, Infinity 98 (59MN7), Performance 96 (59TN6)

No previous code stored - history clear.

Safe to check yourself · checked against manufacturer source

This is an information/status indication rather than a fault.
Follow the user manual - it usually clears on its own or after the indicated action.

Blower running briefly after power-up (normal start sequence).

Safe to check yourself · checked against manufacturer source

This is an information/status indication rather than a fault.
Follow the user manual - it usually clears on its own or after the indicated action.

Limit circuit lockout - limit opened repeatedly; auto-resets after a few hours.

Competent engineer · checked against manufacturer source

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

Ignition lockout - burner failed to light after several attempts; auto-resets after a few hours.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

Blower motor lockout - blower failed to reach speed; auto-resets later.

Competent engineer · checked against manufacturer source

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Gas heating lockout - control will not auto-reset.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Flame detected when the gas valve should be off.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Pressure switch didn't open when the inducer stopped.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Low-voltage (24 V) fuse open.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Model selection / setup error on the control.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

High-heat pressure switch didn't close or reopened.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Low-heat pressure switch didn't close or reopened.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Limit or flame-rollout circuit fault.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Ignition proving failure - no flame signal.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

Blower motor fault.

Competent engineer · checked against manufacturer source

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Inducer (draft) motor fault.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Low-heat pressure switch open while high-heat switch closed.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Control circuitry lockout.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Codes are flashed on the control board's amber LED (short = tens, long = units).

Source: Carrier 355MAV furnace installation/service instructions (Carrier document host) (opens in new tab).

Trane HVAC (US)

Trane Convertible indoor coil with refrigerant leak detection (R-454B, e.g. 5TXC series) 5 codes

Indoor coil · Covers 5TXCB004AS3HCA, 5TXCD010AS3HCA

Green 3 flashes

Active alarm: refrigerant leak detected, or leak-sensor communication/sensor error.

F-gas certified engineer · checked against manufacturer source

Ventilate the room. Do not use naked flames or switch electrical items on/off near the unit if you suspect a leak.
Leave the system off (fans may run automatically to dilute refrigerant - that's normal).
Call an F-gas certified engineer to leak test and check the sensor before restarting.

Green 4 flashes

Past refrigerant-detected alarm stored (shown for up to 30 days).

F-gas certified engineer · checked against manufacturer source

Ventilate the room. Do not use naked flames or switch electrical items on/off near the unit if you suspect a leak.
Leave the system off (fans may run automatically to dilute refrigerant - that's normal).
Call an F-gas certified engineer to leak test and check the sensor before restarting.

Green 5 flashes

Past leak-sensor communication error stored.

Competent engineer · checked against manufacturer source

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Green 6 flashes

Past leak-sensor error stored.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Amber fast blink

Communication (CAN) to the system lost.

Competent engineer · checked against manufacturer source

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Source: Trane Installer's Guide 18-AD45D1-1B-EN (convertible coil with leak detection) (opens in new tab).

Lennox Furnaces & HVAC (US)

Lennox SL280UHVK gas furnace 25 codes

Gas furnace · Covers SL280UHVK, SL280DFV (similar ignition control)

E110

Mains supply voltage low.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

E111

Mains live/neutral reversed.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

E112

No earth (ground) detected.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

E115

24 V control supply low.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

E124

No thermostat signal for over 3 minutes (communicating systems).

Competent engineer · checked against manufacturer source

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

E125

Control self-check / hardware failure.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

E150

A2L refrigerant leak alarm.

F-gas certified engineer · checked against manufacturer source

Ventilate the room. Do not use naked flames or switch electrical items on/off near the unit if you suspect a leak.
Leave the system off (fans may run automatically to dilute refrigerant - that's normal).
Call an F-gas certified engineer to leak test and check the sensor before restarting.

E151

Refrigerant leak sensor 1 fault.

F-gas certified engineer · checked against manufacturer source

Ventilate the room. Do not use naked flames or switch electrical items on/off near the unit if you suspect a leak.
Leave the system off (fans may run automatically to dilute refrigerant - that's normal).
Call an F-gas certified engineer to leak test and check the sensor before restarting.

E200

Hard lockout - flame rollout switch open (or was open).

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E201

Blower motor not communicating with the control.

Competent engineer · checked against manufacturer source

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

E204

Gas valve wiring fault.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

E207

Hot-surface igniter open circuit.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

E223

Low pressure switch failed to close.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E224

Low pressure switch stuck closed.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E225

High pressure switch failed to close.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E227

Low pressure switch opened during ignition or running.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E228

Combustion air inducer calibration failed.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

E240

Flame signal weak while running.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

E241

Flame sensed when it shouldn't be.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

E250

Limit switch circuit open.

Competent engineer · checked against manufacturer source

Check that flow isn't restricted: valves open, filter clean, pump running.
Allow the unit to cool and reset as per the user manual.
If repeated, an engineer must find the cause - do not bypass any limit or safety stat.

E270

Soft lockout - no flame after maximum ignition attempts.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

E290

Igniter circuit fault.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

E292

Blower motor can't start (seized or obstructed).

Competent engineer · checked against manufacturer source

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

E311

Heat output reduced because airflow is restricted (warning).

Safe to check yourself · checked against manufacturer source

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

E312

Restricted airflow in cooling or fan mode (warning).

Safe to check yourself · checked against manufacturer source

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Source: Lennox SL280UHVK installation instructions 508553-02 (opens in new tab).

Rheem Furnaces & HVAC (US)

Rheem/Ruud furnace and heat pump fault flash codes differ between control boards and we could not verify them against a Rheem-hosted manual in this build. Use the code label inside the furnace door or the product documentation link. Rheem residential heating & cooling product documentation (opens in new tab)

Goodman Furnaces & HVAC (US)

Goodman/Amana control-board flash codes differ between board versions and we could not verify them against a Goodman-hosted manual in this build. Use the label inside the blower door or the Literature Library link. Goodman Literature Library (opens in new tab)

Midea Air conditioning

Midea Split / mini-split (Xtreme, Blanc, Mission) 13 codes

Air conditioner · Covers Xtreme Save, Blanc, Mission, Breezeless

E0/EH 00

Indoor EEPROM (memory) fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

E1/EL 01

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E3/EH 03

Indoor fan speed out of control.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

E4/EH 60

Indoor room temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

E5/EH 61

Indoor evaporator sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

EC/EL 0C

Refrigerant leak / low charge detected.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor ambient sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor coil (condenser) sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Compressor discharge sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Inverter IPM module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Over/under-voltage protection.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Compressor top temperature protection.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor drive error.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Newer models show two-part codes (e.g. EH 00); older show E0-style codes.

Source: Midea official owner's manuals (midea.com document library) (opens in new tab).

Gree Air conditioning

Gree Split (Vireo, Livo, Sapphire, Pular) 16 codes

Air conditioner · Covers Vireo, Livo Gen3, Sapphire, Pular, Bora

High pressure protection.

F-gas certified engineer · confirm in your model's manual

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Indoor coil anti-freeze protection.

Safe to check yourself · confirm in your model's manual

Clean or replace the filters and make sure no furniture or curtains block the indoor unit.
Let the coil defrost (switch to fan only for an hour) and check fan speed isn't on its lowest setting.
If it repeats, an F-gas engineer should check for low refrigerant charge.

Low pressure protection / refrigerant shortage.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Compressor discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Overcurrent protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Indoor room temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor evaporator sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor ambient sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Outdoor condenser sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Discharge sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Compressor overload protection.

F-gas certified engineer · confirm in your model's manual

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

IPM module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Indoor fan not reporting speed.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Outdoor fan fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

PH/PL

Supply voltage too high / too low.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

The official manual's code table could not be machine-read in this build.

Source: Gree Comfort owner's & installation manuals (opens in new tab).

Haier Air conditioning

Haier Split (Flexis, Tundra, Pearl) 12 codes

Air conditioner · Covers Flexis Plus, Tundra Plus, Pearl Plus

Indoor room temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor coil temperature sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Indoor EEPROM fault.

Competent engineer · confirm in your model's manual

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Indoor-outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

E14

Indoor fan motor fault.

Competent engineer · confirm in your model's manual

Isolate power and check the filters and the fan area for obstructions (do not open sealed covers).
Engineer to check the fan motor turns freely, its connector, and the feedback/supply from the PCB.

Outdoor IPM module protection.

Competent engineer · confirm in your model's manual

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Outdoor communication fault.

Competent engineer · confirm in your model's manual

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Discharge temperature too high.

F-gas certified engineer · confirm in your model's manual

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Outdoor coil sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Supply over/under voltage.

Competent engineer · confirm in your model's manual

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Outdoor fan motor fault.

Safe to check yourself · confirm in your model's manual

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

F25

Discharge sensor fault.

Competent engineer · confirm in your model's manual

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Source: Haier UK user manuals (opens in new tab).

Vaillant Heat pumps & boilers

Vaillant aroTHERM plus air-to-water heat pump 26 codes

Heat pump · Covers aroTHERM plus 3.5 kW, aroTHERM plus 5 kW, aroTHERM plus 7 kW, aroTHERM plus 10 kW, aroTHERM plus 12 kW

F.22

System water pressure too low.

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

F.42

Coding resistor (unit identity) fault.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

F.103

Spare part identification fault (wrong part fitted).

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

F.514

Compressor inlet temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F.520

Building-circuit flow temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F.532

Building-circuit flow rate too low.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F.536

Compressor outlet temperature out of range (too high).

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.537

Compressor outlet pressure too high.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F.539

Refrigerant pressure too low.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.554

Refrigerant pressure outside the operating window.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.583

Building-circuit flow temperature too low (e.g. during defrost).

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F.703

Low-pressure sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F.718

Fan blocked.

Safe to check yourself · checked against manufacturer source

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

F.729

Compressor outlet temperature too low.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.731

High-pressure switch opened.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F.732

Compressor outlet temperature too high.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.733

Evaporation temperature too low.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

F.737

Condensation temperature too high.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

F.752

Inverter board fault.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F.753

Inverter board not recognised / connection fault.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F.774

Air inlet temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

F.788

Building-circuit pump fault.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F.817

Inverter fault - mains voltage.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

F.1119

Compressor inrush current limiter fault.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

F.1120

Immersion heater phase failure.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

F.9998

Heat pump to controller (eBUS) communication fault.

Competent engineer · checked against manufacturer source

Power-cycle the system at the isolator.
Check the wired controller cable is secure at both ends and not run alongside mains cables.
Engineer to check controller address/master-slave settings and the controller itself.

Source: Vaillant UK - Heat pump fault codes (opens in new tab).

Vaillant ecoTEC boilers (plus, pro, exclusive) 17 codes

Gas boiler · Covers ecoTEC plus 832/835/838, ecoTEC pro 24/28/30, ecoTEC exclusive

F.0

Flow temperature sensor circuit broken.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.1

Return temperature sensor circuit broken.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.10

Flow sensor short-circuited.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.11

Return sensor short-circuited.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.20

Safety thermal cut-out has tripped.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.22

Water pressure too low.

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

F.23

Flow/return temperature difference too big (poor circulation).

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F.24

Temperature rising too fast (poor circulation/air).

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

F.27

Flame signal while burner is off.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

F.28

No flame established during ignition.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

F.29

Flame lost and re-ignition failed.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

F.32

Fan fault.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

F.35

Air/flue pipe blocked.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

F.54

Gas supply problem (with F.28/F.29).

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

F.75

Pressure rise not detected when pump started.

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

F.83

Little or no temperature rise at burner start (low water / air).

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

F.88

Gas valve electrical connection interrupted.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Gas boiler: anything beyond pressure top-up and a single reset is Gas Safe work.

Source: Vaillant UK - Boiler fault codes (opens in new tab).

Grant Heat pumps

Grant Aerona3 R32 air source heat pump (HPID6/10/13/17R32) 20 codes

Heat pump · Covers HPID6R32, HPID10R32, HPID13R32, HPID17R32

Current transformer (CT) disconnected.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Overcurrent protection.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Abnormal compressor speed.

F-gas certified engineer · checked against manufacturer source

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

Suction temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Defrost temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Discharge temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Upper fan motor fault (17 kW model).

Safe to check yourself · checked against manufacturer source

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Outdoor air temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Lower fan motor fault.

Safe to check yourself · checked against manufacturer source

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

Main PCB temperature too high.

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

Main PCB sensor fault.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Main PCB fault.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Controller PCB serial communication fault.

Competent engineer · checked against manufacturer source

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

Main PCB fault after power reset.

Competent engineer · checked against manufacturer source

Power-cycle at the isolator once.
If it returns, the engineer should inspect the PCB for burnt tracks, moisture or loose connectors before replacing it.

Leaving (flow) water temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Return water temperature sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

High pressure switch operated.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Circulating pump fault.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

High pressure switch fault (17 kW model).

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

Compressor overheat protection relay operated (17 kW model).

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

Subset of the outdoor-unit error table in the installation manual.

Source: Grant Aerona R32 installation & servicing instructions (DOC 0136) (opens in new tab).

NIBE Heat pumps

NIBE S2125 air/water heat pump (with VVM/SMO) 18 codes

Heat pump · Covers S2125-8, S2125-12, with VVM S320 / SMO S40

212

Low-pressure too low in cooling mode.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

233

Fan blocked or not connected.

Safe to check yourself · checked against manufacturer source

Isolate power and look for leaves, snow, ice or debris jamming the outdoor fan - clear from outside the guard only.
Make sure nothing is blocking airflow around the outdoor unit (keep roughly 30 cm clear behind and 1 m in front unless the manual says otherwise).
If the fan is free and the code persists, an engineer should test the fan motor and PCB output.

234

Flow and return sensors swapped.

Competent engineer · checked against manufacturer source

This normally follows a repair or board change.
Engineer to check DIP switches / model selection / capacity code against the installation manual.

236

Ten defrosts in a row failed.

Safe to check yourself · checked against manufacturer source

Check the outdoor unit isn't sitting in snow/ice and that defrost water can drain away.
Clear debris from the outdoor coil and keep airflow free.
If defrosts keep failing, an engineer should check the defrost sensor and reversing valve.

237

Compressor running for too short periods.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

238

Repeated high discharge temperatures.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

240

Evaporation temperature too low.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

254

Communication fault with the inverter.

Competent engineer · checked against manufacturer source

Switch the unit off at the isolator, wait a few minutes and power it back up - a one-off glitch often clears.
If it returns: an engineer should check the interconnecting cable for loose, swapped or damaged cores and correct polarity.
Engineer to confirm both boards have power and that the communication signal is present before replacing a PCB.

291

Repeated safety defrosts - poor airflow over the evaporator.

Safe to check yourself · checked against manufacturer source

Check the outdoor unit isn't sitting in snow/ice and that defrost water can drain away.
Clear debris from the outdoor coil and keep airflow free.
If defrosts keep failing, an engineer should check the defrost sensor and reversing valve.

294

Repeated low-pressure alarms.

F-gas certified engineer · checked against manufacturer source

Check the outdoor coil is clean and unobstructed and that the filters indoors are clean - poor airflow mimics refrigerant faults.
Do not top up or open the refrigerant circuit yourself - this is restricted to F-gas certified engineers.
F-gas engineer to leak test, check pressures/superheat and repair the leak before recharging by weight.

295

Repeated high-pressure alarms.

F-gas certified engineer · checked against manufacturer source

Clean the outdoor coil (cooling) or indoor filters/coil (heating) and clear any airflow obstruction.
Check the outdoor fan runs; confirm all service valves are fully open (engineer task).
F-gas engineer to check for overcharge, non-condensables or a blockage in the refrigerant circuit.

314

Unspecified fault - often low system water pressure.

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

328

Mains supply disturbance.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

329

Inverter overheated (poor cooling).

Competent engineer · checked against manufacturer source

Isolate power for at least 10 minutes (inverter capacitors hold charge) and restart once.
Check the heat sink area / outdoor unit is not overheating from blocked airflow.
Engineer to test the inverter PCB, IPM and compressor wiring; do not open the unit while live.

333

Compressor phase missing.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

334

Repeated failed compressor starts.

F-gas certified engineer · checked against manufacturer source

Check the supply voltage is stable and the unit is on a correctly rated breaker.
Power-cycle once; repeated trips need an engineer.
Engineer to check compressor windings, inverter output and refrigerant charge - compressor work requires F-gas certification.

418

Low flow during defrost - filter or pump.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

530-541

Temperature sensor open or short circuit.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Numbers are S-series alarm numbers; VVM/SMO controllers show different numbers for the same alarm.

Source: NIBE S2125 Installer Manual (IHB EN 2125-1) (opens in new tab).

Ideal Heating Boilers

Ideal Heating Logic / Logic+ / Vogue boilers 12 codes

Gas boiler · Covers Logic Max Combi, Logic+ Combi, Logic Heat, Logic System, Vogue Max

Flow overheat or no water circulating.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Flame loss / ignition lockout.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

No water flow.

Safe to check yourself · checked against manufacturer source

Check the system pressure gauge (usually 1-2 bar cold) and top up via the filling loop if low.
Clean the magnetic/strainer filter and make sure isolation valves on the heat pump flow/return are open.
Bleed air from radiators and the system; check enough zones/TRVs are open to give minimum flow.
If it persists, an engineer should check the circulating pump, flow sensor and for blockages.

Boiler reset too many times in a short period.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

False flame lockout.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Low water pressure.

Safe to check yourself · checked against manufacturer source

Read the pressure gauge with the system cold. Top up to the installer's set pressure (often 1-1.5 bar) via the filling loop.
Close the filling loop fully afterwards. If pressure keeps dropping, look for leaks at radiators, valves and the pressure relief discharge pipe.
Repeated loss needs an engineer to check the expansion vessel charge and relief valve.

Flame loss.

Gas Safe registered engineer · checked against manufacturer source

Check other gas appliances work and the gas meter emergency control valve is on; check prepay credit.
In cold weather check the condensate pipe isn't frozen (pour warm - not boiling - water on the outside section).
Reset once as the user manual describes. If it locks out again, call a Gas Safe registered engineer.

Fan fault.

Gas Safe registered engineer · checked against manufacturer source

Do not keep resetting. Check outside that the flue terminal is not blocked.
If you smell gas or suspect fumes, ventilate, turn off at the meter and call the gas emergency number (UK 0800 111 999).
A Gas Safe registered engineer must investigate flue, fan and pressure switch faults.

Flow thermistor fault.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Return thermistor fault.

Gas Safe registered engineer · checked against manufacturer source

Record the code and reset once only.
This relates to gas/combustion components - only a Gas Safe registered engineer may open the appliance.

Outside sensor fault.

Competent engineer · checked against manufacturer source

Note the code and power-cycle once; record whether it returns straight away or after running.
Engineer to check the named thermistor's plug on the PCB and measure its resistance against the manual's table.
Replace the thermistor if open/short circuit; replace the PCB only if the sensor reads correctly.

Low mains voltage.

Competent engineer · checked against manufacturer source

Check the supply: other heavy loads, loose connections in the isolator, or a weak supply can trigger this.
Make sure the unit has its own correctly sized circuit.
Engineer/electrician to measure voltage under load and check terminals for heat damage.

Gas boiler: anything beyond pressure top-up and a single reset is Gas Safe work.

Source: Ideal Heating - Boiler fault codes (opens in new tab).

Related: if there is no code, work through the step-by-step troubleshooting guide; find the installation and service manual with the appliance manuals finder; plan servicing with the preventative maintenance checklist; and check refrigerant figures with the HVAC and refrigeration calculators. Build your skills with Tradefox training courses.

Keep your skills sharp.

Hands-on fault-finding practice, plus more free tools for the trade.

Every HVAC and heating error code, by brand

Daikin Air conditioning & heat pumps

Daikin Split / wall units (Perfera FTXM, Emura FTXJ, Comfora FTXP) 30 codes

Daikin Sky Air (FBA, FCAG, FUA, FHA) indoor + RZAG/RZASG outdoor 30 codes

Daikin Altherma 3 R / H air-to-water heat pump 11 codes

Mitsubishi Electric Air conditioning & heat pumps

Mitsubishi Electric M-Series wall units (MSZ-LN, MSZ-AP, MSZ-HR) 26 codes

Mitsubishi Electric Mr Slim (PEAD/PKA/PLA indoor, PUZ/SUZ outdoor) 26 codes

Mitsubishi Electric SLZ / PLA ceiling cassettes 26 codes

Mitsubishi Electric Ecodan air source heat pump (PUZ-WM / PUHZ-W with FTC controller) 16 codes

Fujitsu (GENERAL) Air conditioning & heat pumps

Fujitsu (GENERAL) Airstage / wall split (ASYG, ASYA, ARYG duct, AUYG cassette) 24 codes

LG Air conditioning & heat pumps

LG Split AC (Artcool, Standard Plus, Dualcool) 25 codes

LG Therma V air-to-water heat pump 9 codes

Samsung Air conditioning & heat pumps

Samsung WindFree / wall split 15 codes

Samsung EHS Mono / Split air-to-water heat pump 7 codes

Panasonic Air conditioning & heat pumps

Panasonic Etherea / TZ / wall split 37 codes

Panasonic Aquarea air-to-water heat pump 17 codes

Toshiba Air conditioning & heat pumps

Toshiba RAS split / Digital Inverter / Seiya / Haori 28 codes

Toshiba SMMS / VRF indoor units (TCC-Link) 28 codes

Hitachi Air conditioning & heat pumps

Hitachi Utopia / RAS split & ducted 23 codes

Carrier Furnaces & HVAC (US)

Carrier 355MAV / Infinity two-stage condensing gas furnace 18 codes

Trane HVAC (US)

Trane Convertible indoor coil with refrigerant leak detection (R-454B, e.g. 5TXC series) 5 codes

Lennox Furnaces & HVAC (US)

Lennox SL280UHVK gas furnace 25 codes

Rheem Furnaces & HVAC (US)

Goodman Furnaces & HVAC (US)

Midea Air conditioning

Midea Split / mini-split (Xtreme, Blanc, Mission) 13 codes

Gree Air conditioning

Gree Split (Vireo, Livo, Sapphire, Pular) 16 codes

Haier Air conditioning

Haier Split (Flexis, Tundra, Pearl) 12 codes

Vaillant Heat pumps & boilers

Vaillant aroTHERM plus air-to-water heat pump 26 codes

Vaillant ecoTEC boilers (plus, pro, exclusive) 17 codes

Grant Heat pumps

Grant Aerona3 R32 air source heat pump (HPID6/10/13/17R32) 20 codes

NIBE Heat pumps

NIBE S2125 air/water heat pump (with VVM/SMO) 18 codes

Ideal Heating Boilers

Ideal Heating Logic / Logic+ / Vogue boilers 12 codes

Keep your skills sharp.

HVAC error code look-up

Apps

Product

For Organisations

Campaigns

Support

About us

Social