Daikin H6-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin H6-00 fault code mean?
The H6-00 fault code indicates that the outdoor unit's inverter drive has lost rotor-position feedback from the compressor motor. To run a variable-speed compressor safely, the inverter PCB continuously tracks the exact rotational position of the motor's rotor so it can energise the correct winding at precisely the right moment. When that feedback signal disappears or returns data that makes no sense, the drive immediately cuts power to the compressor rather than risk firing the wrong winding — which could destroy the motor. The result is that the heat pump cannot start its compressor at all, so no heating or cooling is produced. The '00' suffix identifies which outdoor unit system the fault belongs to in multi-system configurations. This is a serious fault that should not be ignored: running the system repeatedly against this error risks escalating a repairable electrical fault into a full compressor failure.
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
- Loose or corroded compressor wiring connector Common
The three-phase wiring loom between the inverter PCB and the compressor terminals (U, V, W) can work loose over time due to vibration, thermal cycling, or a poor original fit. A high-resistance or intermittent connection breaks the position-feedback loop even if the compressor motor itself is perfectly healthy. Engineers report this is the single most common finding on H6 call-outs.
- Faulty or damaged outdoor inverter PCB Common
The inverter board drives the compressor and interprets the back-EMF rotor-position signals. Moisture ingress, power surge, or component ageing can cause the position-detection circuit on the PCB to fail, producing an H6 error even when the compressor wiring and motor are intact.
- Refrigerant circuit issue causing mechanical overload at startup Sometimes
A shortage or overcharge of refrigerant, a clogged circuit, or a closed service valve can place extreme mechanical load on the compressor at the moment it tries to start. If the motor struggles to turn, rotor-position signals become erratic and the inverter interprets this as a position-detection fault and shuts down.
- Defective compressor motor windings Sometimes
If the compressor motor windings have degraded insulation or an internal short, the back-EMF signal the inverter relies on will be corrupted or absent. This is usually confirmed by a winding-resistance and insulation test performed by an engineer and is a less frequent but more costly finding.
- Supply voltage outside acceptable limits Rare
Sustained under-voltage or over-voltage on the incoming supply can cause the inverter to misread the compressor position signals, triggering H6 even when all hardware is sound. Worth checking the supply quality if faults appear intermittently and no wiring or component fault is found.
How to fix it
- Switch the heat pump off at the controller and leave it off DIY safe
Do not attempt to reset and restart the system repeatedly. Each failed start attempt risks winding heat build-up and can escalate the fault. Power the unit off at the indoor controller and leave it in a safe, non-running state.
- Check for a tripped MCB or isolator at the consumer unit or outdoor disconnect DIY safe
Go to your consumer unit and confirm the circuit breaker for the heat pump has not tripped. If it has tripped, reset it once. If it trips again immediately, do not reset it a second time — leave it off and call an engineer. Also confirm the outdoor isolator (usually mounted on the wall near the outdoor unit) is in the ON position.
- Clear any debris, snow, or ice from around the outdoor unit DIY safe
Ensure the area around the outdoor unit is clear of leaves, dirt, or ice build-up that could be restricting airflow. While this is unlikely to be the direct cause of H6, it is a simple visual check worth doing before calling an engineer. Do not pour hot water on the unit to de-ice it.
- Check the system pressure on the indoor unit or buffer vessel gauge DIY safe
Look at the pressure gauge on your heating system (often on the buffer vessel or the indoor unit itself). Normal operating pressure is typically between 1.0 and 2.5 bar. If it reads below 1.0 bar, top up via the filling loop following your system's manual. Low pressure alone will not cause H6 but is worth noting for the engineer.
- Do not attempt to access the outdoor unit's electrical compartment heat pump engineer
The inverter PCB inside the outdoor unit stores lethal DC voltages in its capacitors that remain present for several minutes after power is removed. Only a qualified engineer with the correct equipment and knowledge of the discharge dwell time should open this panel.
- Contact an MCS-certified heat pump engineer to diagnose and repair the fault heat pump engineer
An engineer will carry out a safe diagnostic sequence: inspecting and re-torquing the U, V, W compressor terminal connections; testing wiring continuity from the PCB to the compressor; checking refrigerant pressures and confirming all service valves are open; measuring compressor winding resistance and insulation with a proper megger tester; and checking incoming supply voltage quality. Depending on findings, the repair will typically be a connector re-termination, PCB replacement, or — less commonly — a compressor swap. Any work involving the refrigerant circuit must be carried out by an F-Gas registered engineer.
Parts you may need
- Outdoor unit inverter PCB (model-specific) · from £320
- Compressor wiring loom / terminal connector set · from £45
- Compressor motor (model-specific, supply only) · from £850
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 £150–£500, depending on the underlying cause.
Frequently asked questions
Can I reset the H6-00 fault myself and keep using the heat pump?
You can try a single reset from the controller, but if H6 returns you should leave the unit off. Repeatedly forcing the compressor to attempt a start against a position-detection fault risks overheating the motor windings and turning what might be a straightforward wiring or PCB repair into a compressor replacement costing well over £1,400. Switch off and call an engineer promptly.
How much does it cost to repair a Daikin H6-00 fault in the UK?
For the most common causes — a loose compressor connector or a failed inverter PCB — most UK homeowners pay between £150 and £500 including call-out, labour, and parts. If the compressor motor itself has failed, replacement costs start at around £1,400 for a small system and rise significantly for larger or multi-split configurations. On older or out-of-warranty systems, it is worth asking your engineer to compare repair cost against the price of a new unit before committing to a compressor swap.
Does H6-00 mean my compressor is definitely broken?
Not necessarily. In many cases the compressor motor is perfectly healthy, and the fault is caused by a loose wiring connector or a failed inverter PCB — both far cheaper fixes. An engineer will test the compressor windings with an insulation tester before recommending a compressor replacement. Statistics from field engineers suggest the compressor itself is the culprit in fewer than half of H6 call-outs.
Do I need an F-Gas engineer for H6-00, or will any heat pump engineer do?
Most H6-00 repairs (wiring, PCB replacement) do not involve opening the refrigerant circuit, so an MCS-certified heat pump engineer is the right person to call. However, if the diagnosis points to a refrigerant shortage, overcharge, or blockage — or if a compressor replacement is needed — the refrigerant circuit must be recovered and recharged, which legally requires an F-Gas registered engineer. Many MCS-certified engineers also hold F-Gas registration, but it is worth confirming before booking.