Daikin F6-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin F6-00 fault code mean?
The F6-00 fault code signals that your Daikin heat pump's outdoor unit has detected abnormally high refrigerant pressure — or that the refrigerant circuit may be overcharged. The 'F6' portion identifies a high-pressure condition in the refrigerant circuit, while '00' confirms the fault is rooted in the outdoor unit. When pressure climbs beyond the safe operating threshold, the unit's protective circuitry shuts things down to prevent damage. This is not a fault to reset and ignore — running the system repeatedly under high-pressure conditions can escalate a relatively modest repair into a far more costly one.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Restricted airflow across the outdoor unit coil Common
Leaves, debris, snow, or a build-up of dirt on the outdoor coil can prevent heat from dissipating properly, causing refrigerant pressure to climb rapidly. This is one of the most common triggers for F6-00 and is worth checking before anything else.
- Outdoor fan not running or running slowly Common
If the fan motor in the outdoor unit has failed, seized, or is only running at reduced speed, the coil cannot reject heat efficiently. The pressure rises quickly as a result. A qualified engineer will need to inspect and test the fan motor.
- Refrigerant circuit overcharged Common
If refrigerant was added without precise measurement — for example, after a leak repair — there may simply be too much refrigerant in the system. An overcharged circuit will push pressures beyond normal limits, particularly during warmer weather or periods of high demand.
- Thermistor disconnection or failure Sometimes
The outdoor air thermistor, heat exchanger deicer thermistor, and liquid pipe temperature thermistor all feed information to the controller. If any of these sensors is disconnected or faulty, the system can misread operating conditions and allow pressures to rise unchecked.
- Blocked or malfunctioning electronic expansion valve (EEV) Sometimes
The EEV regulates refrigerant flow around the circuit. If it sticks closed or partially blocked, refrigerant backs up and pressure builds on the high side. This requires an F-Gas registered engineer to diagnose and rectify.
- Refrigerant leak causing partial blockage or incorrect charge Sometimes
A slow leak can result in an unbalanced charge. Combined with other factors such as a partially blocked filter drier, this can create localised high-pressure conditions even though the overall charge appears normal.
- Faulty outdoor unit PCB Rare
The main PCB in the outdoor unit controls the compressor, fan, and expansion valve. If it is sending incorrect signals — or failing to respond to pressure sensor readings — high-pressure conditions can develop without the normal protective response kicking in correctly.
How to fix it
- Stop using the heat pump immediately DIY safe
Do not attempt to reset and restart the unit repeatedly. Each time the compressor runs under abnormal high-pressure conditions, it risks serious internal damage. Switch the unit off at the controller and leave it off while you work through the following checks.
- Clear debris, snow, or ice from around the outdoor unit DIY safe
Walk around the outdoor unit and remove any leaves, garden debris, snow, or accumulated ice that may be blocking the coil or restricting airflow. Ensure there is at least one metre of clear space around the unit and that nothing is obstructing the fan outlet at the top. Do not use a pressure washer — gently clear what you can by hand or with a soft brush.
- Check that the outdoor fan is visible spinning when the unit is briefly powered DIY safe
If you feel safe doing so, power the unit on briefly from the controller and observe whether the outdoor fan starts. If the fan is not spinning, or is turning sluggishly, switch the unit off and do not continue running it. Report this to your engineer — a failed fan is a likely cause of the high-pressure trip.
- Check your heat pump controller for any secondary fault codes DIY safe
Some Daikin Altherma controllers display a fault history. Note down any additional codes shown alongside F6-00, as these can help your engineer identify whether a thermistor or PCB is also involved. Take a photograph if possible.
- Contact an MCS-certified heat pump engineer for full diagnosis heat pump engineer
All refrigerant-side work — including measuring operating pressures, checking superheat and subcooling, inspecting the EEV, testing thermistors, and adjusting the refrigerant charge — must be carried out by a qualified engineer. Any work on the refrigerant circuit specifically requires an F-Gas registered engineer. Do not attempt to release or add refrigerant yourself. Ask your engineer to inspect the coil, fan motor, thermistor wiring, EEV, and outdoor PCB as part of the diagnostic process.
- Request a full operational test once the fault has been corrected heat pump engineer
After any repair, the engineer should run the system through a complete cycle and confirm that the F6-00 code does not return under normal load. This is also a good opportunity to ask whether an annual service is overdue — blocked coils and low refrigerant charge are issues that routine servicing catches early.
Parts you may need
- Outdoor air thermistor · from £35
- Liquid pipe temperature thermistor · from £35
- Heat exchanger deicer thermistor · from £35
- Electronic expansion valve (EEV) assembly · from £180
- Outdoor unit fan motor · from £220
- Outdoor unit main PCB · from £380
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £180–£550, depending on the underlying cause.
Frequently asked questions
Can I reset the Daikin F6-00 code and keep using my heat pump?
We strongly advise against it. F6-00 indicates that the refrigerant circuit reached an unsafe pressure level, and the unit shut down to protect itself. Repeatedly resetting and running it risks damaging the compressor — which can cost well over £1,400 to replace on a residential unit, compared with perhaps £180–£350 for a sensor swap caught early. Switch the unit off and get an engineer to investigate before restarting.
Do I need an F-Gas engineer, or will any heat pump engineer do?
For any work involving the refrigerant circuit — checking pressures, leak testing, removing or adding refrigerant — the engineer must hold a valid F-Gas qualification (also referred to as a refrigerant handling certificate). An MCS-certified heat pump installer may or may not hold F-Gas credentials, so it is worth confirming before booking. General checks such as thermistor replacement or PCB testing do not require F-Gas certification, but in practice most heat pump engineers who attend refrigerant-related faults will hold both.
Could a dirty coil really cause F6-00, or is it always a refrigerant problem?
A blocked or heavily soiled outdoor coil is one of the most common causes of high-pressure trips on Daikin heat pumps — and it is often overlooked. If heat cannot transfer away from the coil efficiently, pressure builds in exactly the same way as it would with an overcharge. An engineer will check airflow and coil condition before drawing conclusions about the refrigerant charge. Sometimes a thorough coil clean resolves the fault without any refrigerant work at all.
Will this fault affect my Daikin warranty?
Daikin's standard residential warranty covers component faults for three years from installation, provided the unit has been serviced annually by a qualified engineer. The key word is 'annually' — if you cannot produce service records, Daikin may decline the claim. Refrigerant top-ups resulting from a leak are generally treated as a wear item and are not covered under warranty. If your unit is still within warranty, contact your installing engineer or Daikin's UK service line before authorising any third-party repair.