How to Use Daikin Fault Codes

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Confirm the Daikin model

Before diagnosing a fault code, confirm the indoor unit model, outdoor unit model, controller type and system type. Daikin split systems, ducted units, SkyAir, VRV, package units and multi-split systems may use similar fault codes, but the exact checks can vary.

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Record the active fault code

Record the exact fault code shown on the controller, central controller, BMS, service checker or outdoor unit display. If multiple faults are present, write down all codes and the order they appeared where possible.

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Check whether the fault is active or historical

Some Daikin systems store previous fault codes. A historical code can be useful, but it may not be the current reason the unit is stopped. Confirm whether the fault is currently active before replacing parts.

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Check the basics first

Before replacing boards or major parts, check power supply, isolators, fuses, communication wiring, plug connections, controller settings, filters, airflow, water flow, drain condition, refrigerant condition and recent site works.

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Do not reset safety faults without checking the cause

Faults such as high pressure, low pressure, compressor overload, inverter faults, drain faults and repeated communication faults should be investigated before being reset. Repeated resets without fixing the cause can lead to equipment damage.

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Common Daikin Fault Code Checks

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Daikin U4 fault code

A Daikin U4 fault commonly relates to communication between indoor and outdoor units. First checks should include indoor and outdoor power supply, interconnecting wiring, polarity, terminals, controller wiring, outdoor PCB power, indoor PCB power, address settings and signs of recent electrical or installation work.

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Daikin U0 fault code

A Daikin U0 fault is commonly associated with refrigerant shortage or low refrigerant condition. First checks should include leak inspection, operating pressures, superheat, subcooling, stop valves, pipe temperature sensors, expansion valve operation and whether the system has recently been repaired or recharged.

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Daikin U2 fault code

A Daikin U2 fault commonly relates to power supply voltage problems, voltage drop or main circuit overvoltage. First checks should include incoming voltage, phase balance, loose terminals, isolator condition, supply interruptions, power quality and outdoor unit PCB power supply.

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Daikin UA fault code

A Daikin UA fault commonly relates to indoor and outdoor unit combination, model mismatch, setting issue or system configuration problem. First checks should include connected indoor unit count, model compatibility, refrigerant type compatibility, address settings and whether any PCB or controller has recently been replaced.

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Daikin A3 fault code

A Daikin A3 fault commonly relates to the drain level control system. First checks should include condensate drain blockage, drain pump operation, float switch, drain pan level, pipe fall, kinked drain hose, dirty pump, water leaks and plug connections.

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Daikin A5 fault code

A Daikin A5 fault commonly relates to freeze protection or high-pressure control depending on model and operating mode. First checks should include dirty filters, indoor coil condition, indoor fan operation, airflow restriction, coil temperature sensor, low load, refrigerant condition and system operating mode.

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Daikin A6 fault code

A Daikin A6 fault commonly relates to the indoor fan motor. First checks should include fan wheel condition, fan motor plug, motor harness, seized fan, indoor PCB output, obstructions, motor feedback signal and whether the fan runs freely by hand when safely isolated.

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Daikin C4 fault code

A Daikin C4 fault commonly relates to a heat exchanger temperature sensor fault. First checks should include thermistor resistance, sensor plug connection, cable damage, sensor location, water ingress, indoor PCB input and whether the sensor reading is realistic.

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Daikin C9 fault code

A Daikin C9 fault commonly relates to a suction air temperature sensor fault. First checks should include sensor resistance, return air sensor location, plug connection, cable damage, indoor PCB input and whether the measured return air temperature is believable.

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Daikin E3 fault code

A Daikin E3 fault commonly relates to high pressure protection. First checks should include condenser airflow, outdoor fan operation, dirty condenser coil, blocked airflow path, closed service valves, overcharge, non-condensables, pressure transducer, high pressure switch and ambient conditions.

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Daikin E4 fault code

A Daikin E4 fault commonly relates to low pressure protection. First checks should include low refrigerant charge, refrigerant leaks, closed stop valves, blocked filters, poor evaporator airflow, expansion valve operation, restrictions, low load and low ambient operation.

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Daikin E5 fault code

A Daikin E5 fault commonly relates to compressor overload or compressor motor protection. First checks should include compressor current, supply voltage, phase balance, compressor winding resistance, refrigerant pressures, mechanical load, inverter condition and signs of repeated short cycling.

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Daikin E6 fault code

A Daikin E6 fault commonly relates to compressor startup failure. First checks should include compressor wiring, inverter output, compressor winding resistance, supply voltage, stop valves, refrigerant pressure equalisation, compressor mechanical condition and outdoor PCB/inverter PCB condition.

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Daikin E7 fault code

A Daikin E7 fault commonly relates to outdoor fan motor fault. First checks should include outdoor fan blade, fan motor plug, motor harness, seized motor, outdoor PCB output, fan feedback signal, obstructions and condenser coil condition.

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Daikin F3 fault code

A Daikin F3 fault commonly relates to high discharge pipe temperature. First checks should include refrigerant charge, airflow, condenser condition, compressor cooling, discharge pipe thermistor, expansion valve operation, restrictions, non-condensables and system load.

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Daikin H6 fault code

A Daikin H6 fault commonly relates to compressor position detection or compressor feedback. First checks should include compressor wiring, inverter PCB, compressor resistance, insulation resistance, supply voltage and whether the compressor is mechanically locked.

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Daikin H9 fault code

A Daikin H9 fault commonly relates to outdoor air temperature sensor fault. First checks should include outdoor thermistor resistance, plug connection, sensor location, cable damage, water ingress and outdoor PCB input.

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Daikin J3 fault code

A Daikin J3 fault commonly relates to discharge pipe temperature sensor fault. First checks should include thermistor resistance, plug connection, sensor contact, cable damage, sensor location and outdoor PCB input.

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Daikin J6 fault code

A Daikin J6 fault commonly relates to outdoor heat exchanger temperature sensor fault. First checks should include sensor resistance, plug connection, sensor position, cable damage, coil condition and whether the temperature reading is realistic.

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Daikin L4 fault code

A Daikin L4 fault commonly relates to high temperature at the inverter circuit heatsink. First checks should include outdoor unit ventilation, inverter heatsink condition, fan operation, blocked condenser, ambient temperature, outdoor PCB cooling and loose electrical connections.

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Daikin L5 fault code

A Daikin L5 fault commonly relates to inverter output overcurrent. First checks should include compressor winding resistance, compressor insulation resistance, inverter PCB, compressor wiring, supply voltage, stop valves and refrigerant system pressure conditions.

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Daikin P4 fault code

A Daikin P4 fault commonly relates to inverter circuit heatsink temperature sensor fault. First checks should include thermistor resistance, plug connection, inverter PCB, heatsink condition, wiring damage and outdoor unit airflow.

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First Checks Before Replacing Parts

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Check power supply

Confirm the indoor and outdoor units have correct power supply. Check isolators, circuit breakers, fuses, terminals, phase rotation where applicable, voltage drop and signs of loose or burnt connections.

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Check communication wiring

Many Daikin faults are caused by communication wiring issues. Check cable type, polarity, terminals, loose connections, damaged cable, water ingress, incorrect joins, noise interference and wiring changes made during recent site works.

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Check airflow

For pressure, temperature and freeze protection faults, check filters, indoor fan operation, condenser fan operation, coil cleanliness, blocked grilles, closed dampers, fan speed, belt condition and BMS commands.

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Check condensate drains

For drain-related faults, check the drain pan, float switch, drain pump, drain hose, pipe fall, blockages, kinks and whether water is backing up into the unit.

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Check refrigerant condition

For low pressure, high pressure, discharge temperature and refrigerant shortage faults, check refrigerant charge, leaks, stop valves, pipe temperatures, superheat, subcooling, expansion valve operation and coil condition.

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Check sensors and thermistors

Thermistor faults can be caused by failed sensors, poor plug connections, damaged cables, incorrect sensor location or unrealistic readings caused by airflow or water flow issues.

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Check recent site changes

Ask whether any recent electrical work, BMS work, controller replacement, PCB replacement, refrigerant work, cleaning, power outage or tenancy works occurred before the fault appeared.

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Daikin VRV Fault Finding Notes

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Daikin VRV systems can have multiple indoor units, branch selector boxes, central controllers, outdoor modules and BMS interfaces. A single fault code may stop one indoor unit, a group of units or the full system depending on the fault type and system configuration.

When fault finding Daikin VRV systems, check communication wiring carefully. U-series faults can be caused by wiring errors, address duplication, incorrect system setup, controller issues, powered-down indoor units, outdoor PCB faults or problems on another connected unit.

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Do not assume the displayed indoor unit is always the root cause. On larger VRV systems, another indoor unit, branch selector, outdoor unit or control device may be causing the communication or system fault.

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Daikin Ducted and Split System Notes

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For Daikin ducted and split systems, common site issues include dirty filters, blocked indoor coils, blocked outdoor coils, failed fan motors, drain pump faults, poor condensate pipe fall, low refrigerant charge, storm or power-related PCB faults, and communication wiring faults between the indoor and outdoor unit.

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Always check the simple items before replacing expensive components. Many faults can be caused by airflow restriction, poor maintenance, loose plugs, blocked drains, low voltage or incorrect installation wiring.