Daikin 8F-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 8F-00 fault code mean?
The 8F-00 code tells you that the heat pump's control system has lost confidence in the reading from the second suction pipe temperature sensor used during heating on System 1. This thermistor monitors refrigerant conditions at the suction side of the circuit; when the controller detects a signal that is absent, erratic, or outside the expected range, it raises this alarm and prevents the system from running heating. On multi-system Daikin units, code 8E covers System 1's first suction thermistor and 8F covers the second — on a single-system installation, 8F still refers specifically to this secondary sensor on System 1.
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 or drifted suction pipe thermistor Common
The thermistor itself is the most common culprit. Over time, the sensor element can develop an open circuit, a short, or drift far enough out of calibration that the PCB rejects its signal entirely. A failed thermistor will produce a fixed resistance reading or no reading at all, triggering the fault immediately on start-up.
- Loose, corroded or damaged wiring connector Common
The connector linking the thermistor to the outdoor unit PCB is exposed to outdoor conditions — moisture ingress, vibration and thermal cycling can all loosen the crimp or corrode the pins. A poor connection produces an intermittent or out-of-range signal that the controller interprets as a sensor fault, even when the thermistor itself is still functional.
- Defective outdoor unit PCB Sometimes
If both the thermistor and its wiring check out correctly, the fault can lie with the main PCB in the outdoor unit. The signal-conditioning circuitry that interprets the thermistor's resistance can fail, meaning the board misreads or completely ignores a healthy sensor. This is less common but becomes more likely on older units or after a power surge.
How to fix it
- Confirm the active fault code on your controller DIY safe
On a Daikin wired remote or indoor controller, press and hold the CANCEL button for approximately five seconds until the display shows '00' and the unit beeps. Press CANCEL repeatedly to step through stored codes — a long beep indicates the active fault. Note the code (8F-00), then press CANCEL once more to exit diagnostic mode. This confirms the fault before calling an engineer.
- Check the outdoor unit isolator and any tripped MCBs DIY safe
Head to your consumer unit and confirm that the heat pump circuit breaker has not tripped. Also check that the outdoor unit's local isolator switch (usually mounted on the wall beside the unit) is in the ON position. A brief power interruption sometimes causes a fault to latch; if everything looks normal, switch the unit off at the isolator, wait two minutes, then switch back on to attempt a reset. If the 8F-00 code returns, the underlying fault is genuine and requires an engineer.
- Clear any debris or obstructions from around the outdoor unit DIY safe
While this step will not resolve a thermistor fault, it is worth doing before the engineer arrives. Remove any leaves, snow, ice or other debris from the outdoor unit's casing, fan grille and the area immediately around it. This ensures good airflow and rules out any secondary issues that could complicate the engineer's diagnosis.
- Have an engineer inspect and reseat the thermistor wiring and connectors heat pump engineer
An MCS-certified engineer will open the outdoor unit and locate the suction pipe thermistor wiring harness. They will check each connector for signs of corrosion, pin damage or a loose crimp, reseat any suspect connections and clear any moisture. If the harness itself is damaged, it will be replaced. After reseating, the engineer will clear the fault code and test the unit to see whether the error returns.
- Test the thermistor resistance and replace the sensor if faulty heat pump engineer
Using a multimeter, the engineer will measure the thermistor's resistance and compare it against Daikin's reference values at the current ambient temperature. A reading that is open-circuit, short-circuit or significantly outside the expected range confirms the sensor has failed. Replacement involves fitting a new, compatible Daikin thermistor, clipping it correctly to the suction pipe and reconnecting the wiring harness before retesting.
- Inspect and replace the outdoor unit main PCB if required heat pump engineer
If the thermistor and all wiring test as good, the fault lies with the outdoor PCB's ability to process the sensor signal. The engineer will carry out further electrical checks on the board before condemning it. PCB replacement on a Daikin outdoor unit is a specialist task involving careful handling of the refrigerant-circuit connections and correct programming of the new board to match the installation.
- Contact an MCS-certified heat pump engineer heat pump engineer
Because every step beyond the basic checks above involves working inside the outdoor unit — and potentially near the refrigerant circuit — you should book an MCS-certified heat pump engineer. If the fault is traced to a refrigerant-side component, only an F-Gas registered engineer is legally permitted to handle the sealed circuit. Daikin's UK Approved Service Partner network can typically source thermistors and PCBs next-day from the UK distribution centre, so most 8F-00 faults are resolved on the first or second visit.
Parts you may need
- Daikin suction pipe thermistor (System 1, secondary) · from £35
- Outdoor unit wiring harness / connector set · from £25
- Daikin outdoor unit main PCB · from £320
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–£350, depending on the underlying cause.
Frequently asked questions
Can I keep using my Daikin heat pump while the 8F-00 code is showing?
No. The 8F-00 fault will lock out heating mode because the system can no longer monitor suction pipe conditions reliably. Running the heat pump without this protection risks the compressor operating outside safe parameters. Leave the unit off and book an engineer promptly — particularly important in cold weather when you need heating.
Is the 8F-00 fault covered under my Daikin warranty?
It may be. Daikin's standard residential warranty covers component faults for three years (some extended schemes offer longer), provided the unit has been serviced annually by an F-Gas certified engineer and you have the service records to prove it. Wear-and-tear failures — such as a thermistor that has simply reached the end of its service life — are typically not covered. Contact your installer or a Daikin Approved Service Partner with your installation and service paperwork before authorising any repair.
How much does it cost to fix an 8F-00 fault in the UK?
The majority of 8F-00 faults come down to a failed thermistor or a loose connector. A sensor swap including engineer call-out, parts and labour typically costs £180–£350. If the outdoor PCB turns out to be the cause, expect to pay £450–£900 for the board plus labour. These are the two scenarios that cover the vast majority of cases. In rare situations where a refrigerant-circuit component contributes to the fault, costs can be higher — your engineer will advise once they have diagnosed the root cause.
How can I prevent the 8F-00 fault from recurring?
Annual servicing by an MCS-certified, F-Gas registered engineer is the single most effective measure. During a service, the engineer will check all thermistor readings and connections, clean the outdoor unit coil and fan, and verify refrigerant pressures — catching sensor degradation before it causes a lockout. Keeping the outdoor unit clear of debris and ensuring good airflow around it year-round also reduces stress on the system's components.