Daikin 8H-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 8H-00 fault code mean?
The 8H-00 code (also displayed as H8 or H8-00 on Daikin controllers) indicates that the outdoor unit's current transformer (CT) has detected an abnormal reading from the compressor input circuit. Internally, the system compares the compressor's running frequency against the CT-measured input current — when these fall outside expected limits, the unit locks out and raises this fault. The H8 series points squarely at the outdoor unit's inverter electrical system: typically a failed power transistor, a defective reactor, faulty inverter wiring, or a failed outdoor PCB. This is not a homeowner-serviceable fault; the inverter electronics involved carry dangerous voltages and must only be handled by a qualified engineer.
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
- Failed power transistor Common
The power transistor within the outdoor unit's inverter module is the most frequently implicated component when H8 appears. It controls the switching of current to the compressor, and when it degrades or fails, the CT readings fall outside the expected window relative to compressor frequency. A faulty transistor can also accelerate wear on the compressor if the unit is repeatedly reset and run without a proper repair.
- Defective outdoor unit PCB (inverter board) Common
The outdoor unit's printed circuit board processes the CT signal and compares it against operating parameters. A board fault — whether caused by a surge, moisture ingress, age, or component failure — can cause incorrect readings or prevent the inverter from functioning correctly, triggering H8. PCB faults often appear alongside or are confused with transistor failures since they sit within the same inverter assembly.
- Faulty inverter wiring or connection Sometimes
Loose, corroded, or damaged wiring within the inverter circuit can disrupt the CT signal or interrupt current flow to the compressor input. Vibration over time, rodent damage, or a poorly-made connection during a previous service visit are all realistic causes. This is worth ruling out first as it is the most straightforward repair if identified early.
- Defective reactor Sometimes
The reactor (also called a choke or inductor) smooths the inverter's AC input current before it reaches the compressor. If the reactor develops an open circuit, a short, or significant degradation, the current profile seen by the CT will be abnormal and H8 is triggered. Reactor failures are less frequent than transistor or PCB faults but do occur, particularly on older units.
- Genuine CT sensor failure Rare
In some cases the current transformer itself is the source of the problem, reporting false or out-of-range values to the PCB even when the rest of the inverter circuit is functioning correctly. This is the least common root cause but should be considered if all other components test within specification.
How to fix it
- Switch the heat pump off at the controller and wait two minutes before attempting a restart DIY safe
A one-off transient spike can occasionally cause a nuisance H8 trip. Power the unit off fully at the indoor controller, wait at least two minutes to allow the inverter capacitors to discharge their stored charge, then restart. If the fault clears and does not return within a normal heating cycle, monitor the unit over the following 24–48 hours. If the fault reappears, do not continue resetting — repeated resets with an underlying inverter fault risk compressor damage.
- Check that the outdoor unit isolator and any MCBs in the consumer unit are in the ON position DIY safe
An intermittent power supply or a tripped MCB can produce unusual inverter readings. Locate the dedicated circuit breaker for the heat pump in your consumer unit and confirm it has not tripped. Also check the outdoor isolator (usually a red rotary switch mounted near the outdoor unit) is fully in the ON position. Reset any tripped breaker once only — if it trips again, stop and call an engineer.
- Clear any obstructions around the outdoor unit DIY safe
While not a direct cause of H8, heavy snow accumulation, leaf build-up, or debris blocking the outdoor unit's airflow can cause the system to work harder and may compound electrical stress on inverter components. With the unit powered off, clear any debris from around the casing and ensure at least 30 cm of clearance on all sides. Do not insert anything into the unit or touch the fan blades.
- Do not attempt to access the outdoor unit's electrical panel or inverter components heat pump engineer
The inverter system inside the outdoor unit contains capacitors that can hold lethal voltages even after the unit has been switched off. Power transistors, PCBs, reactors, and wiring within this compartment must only be inspected and handled by a suitably qualified engineer. Attempting DIY access risks serious electric shock and will void your warranty.
- Contact an MCS-certified heat pump engineer — refrigerant-adjacent work requires an F-Gas registered engineer heat pump engineer
An engineer will carry out a structured diagnostic of the inverter circuit: checking wiring continuity and connections first, then testing the power transistor, reactor, and PCB in sequence. Based on findings, the faulty component will be replaced. Parts such as power transistors, reactors, and outdoor PCBs must be sourced as genuine Daikin components to preserve warranty validity. If your unit is within its warranty period (three years residential, five years commercial for most Daikin products), contact your installing engineer or Daikin directly before authorising any out-of-warranty repair — keep all previous service records to hand as Daikin requires evidence of annual servicing to honour warranty claims.
Parts you may need
- Daikin outdoor unit inverter PCB · from £320
- Daikin power transistor module · from £180
- Daikin inverter reactor (choke) · from £95
- Daikin compressor input CT sensor · from £55
- Inverter wiring harness (outdoor unit) · 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 £200–£650, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my heat pump while the 8H-00 fault is showing?
No — you should not continue running the unit with an active H8 fault. The fault indicates a problem within the inverter's power electronics, and repeatedly resetting and running the system without addressing the root cause places abnormal stress on the compressor. What might start as a £300–£600 transistor or PCB repair can escalate into a compressor replacement costing £1,400 or more if the underlying fault is ignored. Switch the unit off and arrange for an engineer promptly.
Can a homeowner reset the H8 fault themselves, or does it need an engineer?
You can attempt a single reset by turning the unit off at the controller, waiting two minutes, and restarting — this is worth trying in case the fault was caused by a one-off power anomaly. However, if the fault returns after one restart cycle, further resets will not fix the problem and may cause additional damage. At that point an MCS-certified heat pump engineer needs to carry out a proper inverter diagnostic. Do not attempt to open the outdoor unit or touch any internal components.
How much does it typically cost to repair a Daikin H8 fault in the UK?
For the most common repairs — inverter wiring faults, power transistor replacement, or reactor replacement — most UK homeowners pay in the region of £200 to £650 including parts and labour. An outdoor PCB replacement tends to sit at the higher end of that range or just above it (£450–£900). A full compressor replacement, which is only needed if the unit has been run extensively with a damaged inverter, can cost £1,400 or more — this is why early diagnosis matters.
My Daikin heat pump is still under warranty — will H8 be covered?
It may be, depending on the cause. Daikin's standard residential warranty covers manufacturer component defects for three years (some schemes extend this). However, Daikin requires evidence of annual servicing by an F-Gas certified engineer to validate warranty claims — if your service records are incomplete, a claim may be rejected. Contact your original installing engineer or Daikin's UK support line before authorising any repairs, and do not allow an unauthorised engineer to open the outdoor unit as this can void coverage.