Daikin H0-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin H0-00 fault code mean?
The H0-00 code indicates that the heat pump's outdoor unit has detected an abnormal reading from the voltage or current sensor on the compressor side. This can be triggered by power supply conditions falling outside acceptable limits — either too high or too low — or by the compressor failing to draw current when it should have started. The unit will shut down to protect the compressor, though the outdoor and indoor fans may continue running. If the fault condition is detected four times without a successful compressor run of 60 or more accumulated minutes in between, the unit locks out completely.
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
- Faulty outdoor inverter or main PCB Common
The inverter PCB controls the compressor's speed and monitors its current draw. If the board's onboard voltage regulators or current-sensing components degrade or fail, the unit can no longer verify that the compressor is operating correctly and raises H0-00. This is one of the most frequently reported root causes by UK heat pump engineers attending this code.
- Loose, damaged or corroded wiring connector Common
Vibration from normal operation, thermal cycling, or moisture ingress can cause connectors on the harness between the main control board and the compressor or sensors to back out or corrode. Even a marginally poor contact on a sensor feed can produce erratic readings that trigger this fault.
- Mains supply voltage out of range Sometimes
The UK grid should deliver 230 V ±10%. If the incoming supply voltage at the outdoor unit terminals is consistently high or low — due to a weak grid connection, a long cable run, or a failing isolator — the control board's self-check on compressor power can fail even when all components are perfectly healthy.
- Compressor winding or start-up failure Sometimes
A compressor whose windings have degraded will either draw no current on start-up or draw it in a pattern the inverter board cannot recognise as normal. This triggers the current-sensor alarm. Repeated short-cycling on this code without investigation can accelerate compressor wear significantly.
- Faulty temperature sensor (thermistor) sending corrupt data Rare
On Altherma models in particular, thermistor R5T and related sensors feed into the board's start-up decision logic. A thermistor with a drifted resistance value can confuse the PCB into believing compressor conditions are unsafe, causing H0-00 even when the compressor itself is mechanically sound.
How to fix it
- Switch the unit off and perform a single controlled power cycle DIY safe
Turn the heat pump off at the indoor controller, then isolate it at the outdoor unit's local isolator switch or at the consumer unit MCB. Leave it fully de-energised for at least 10 minutes — this allows the inverter capacitors to discharge and the control boards to reset fully. Restore power and observe whether the unit starts normally. If H0-00 returns within the first few minutes of operation, do not attempt a second or third reset; repeated resets on a compressor-related fault can accelerate internal damage.
- Check the MCB and isolator for signs of a tripped or weak connection DIY safe
Inspect the MCB in your consumer unit that feeds the heat pump, and the local isolator at the outdoor unit. If either has tripped, reset it once. If it trips again immediately, do not force it — call an engineer. Look for any signs of heat discolouration or corrosion at the isolator terminals, as a poor mains connection can cause the under-voltage condition that triggers this fault.
- Clear the area around the outdoor unit DIY safe
Ensure there is no debris, compacted snow, ice or vegetation blocking airflow around the outdoor unit. While this is unlikely to be the direct cause of H0-00, a severely restricted unit can cause electrical stress on start-up. Keep at least 300 mm clearance on all sides and 500 mm at the air outlet.
- Have an engineer inspect all wiring harnesses and PCB connectors in the outdoor unit heat pump engineer
An MCS-certified engineer will isolate the unit, remove the outdoor unit cover and systematically reseat every connector on the main PCB and inverter board. They will check for backed-out pins, corrosion, chafed insulation and heat damage — all common causes of intermittent sensor faults on Daikin outdoor units. This is not homeowner-safe work due to stored charge in inverter capacitors even after isolation.
- Have an engineer check and measure the mains supply voltage at the outdoor unit terminals heat pump engineer
The engineer will verify that incoming voltage at the outdoor unit terminals is within the acceptable range for the model. They will also check the control board's internal supply rails — the logic board typically requires a stable 5 VDC rail; a reading below approximately 4.75 VDC points to a failed onboard regulator and board replacement will be required.
- Have an engineer test the thermistors, including R5T on Altherma models heat pump engineer
Using a multimeter, the engineer will measure resistance across each relevant thermistor at a known ambient temperature and compare readings against Daikin's resistance-temperature tables. A sensor reading outside the expected range will be replaced. On Altherma units the engineer should also confirm the main board's status LED is blinking at its normal regular interval, indicating the board's processor is healthy.
- Have an engineer test the compressor windings and inverter output heat pump engineer
The engineer will carry out insulation resistance and winding resistance checks on the compressor. They will also check that the inverter board is producing balanced three-phase output to the compressor terminals. An unbalanced or absent output phase confirms inverter PCB failure; abnormal winding readings confirm compressor wear or failure.
- Replace the faulty component — PCB, sensor or compressor — as diagnosed heat pump engineer
Based on the above checks the engineer will replace whichever component has failed. PCB replacement on a Daikin outdoor unit requires careful programming and configuration to match the unit's model variant. Compressor replacement is a major job requiring F-Gas registration as the refrigerant circuit must be recovered, the compressor swapped, and the system re-charged and leak-tested before returning it to service.
- Contact an MCS-certified heat pump engineer if you have not already done so heat pump engineer
H0-00 is not a fault that resolves itself. If a single power cycle has not cleared the code, the unit needs professional diagnosis. Continuing to run or repeatedly reset the unit risks turning a relatively straightforward PCB or sensor repair into a full compressor replacement. Book an MCS-certified engineer promptly; ensure any refrigerant circuit work is carried out by an F-Gas registered engineer.
Parts you may need
- Daikin outdoor unit main PCB / inverter board · from £380
- Compressor current/voltage sensor harness assembly · from £65
- Thermistor sensor (R5T or equivalent, Altherma series) · from £35
- Outdoor unit local isolator switch · from £45
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 H0-00 code myself and carry on using the heat pump?
You can attempt a single controlled power cycle — off at the isolator for 10 minutes, then back on — to rule out a transient voltage glitch. If the code clears and the unit runs normally for several hours, monitor it closely. However, if H0-00 returns, do not keep resetting it. This code specifically relates to the compressor's sensor system, and running through a compressor-related fault repeatedly can escalate a £200–£300 sensor or wiring repair into a compressor replacement costing well over £1,500. Switch off and call an MCS-certified engineer.
How much does it typically cost to fix a Daikin H0-00 fault in the UK?
For the most common causes — a wiring or connector fault, a failed thermistor, or a PCB repair — most UK homeowners pay somewhere between £180 and £550 including parts and labour. If the outdoor inverter or main PCB needs replacing, expect the upper end of that range. In a small number of cases the compressor itself has failed, which is a significantly larger job; costs for compressor replacement on a residential Daikin Altherma typically start at around £1,400 and can go higher on larger systems, so it is worth getting a diagnosis quickly before the fault worsens.
Why are the fans still running if the compressor has stopped on H0-00?
This is normal behaviour. When the Daikin control board detects an abnormal compressor sensor reading it cuts power to the compressor as a protective measure, but the indoor and outdoor fans are on a separate circuit and may continue to run. The fans circulating air without the compressor operating means your home will not be heated or cooled, but the continued fan operation is not a sign that the unit is working — it simply reflects how Daikin's fault-handling logic is designed.
Does H0-00 affect my Daikin warranty?
It can do, depending on the root cause. Daikin's residential warranty typically covers manufacturing defects in parts for three years from installation, provided the unit has been serviced annually by an appropriately qualified engineer and installed by an MCS-certified installer. If the PCB or compressor has failed due to a manufacturing defect within the warranty period, the repair may be covered. However, Daikin requires proof of annual servicing to honour warranty claims — if you do not have service records, the claim is likely to be rejected. Contact your installing engineer or Daikin's UK support line before commissioning any paid repairs.