Daikin C5-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin C5-00 fault code mean?
The C5-00 fault code indicates that the indoor unit's refrigerant pipe thermistor — the temperature sensor that monitors the heat exchanger on the refrigerant outlet side — is reporting a reading outside its expected range. In Daikin's fault-code structure, C-series codes always point to sensor faults within the indoor unit, and the -00 suffix identifies the first (or only) indoor unit affected. Until the sensor signal returns to normal, the heat pump will suspend normal operation to protect itself from running on unreliable temperature data.
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 out-of-range indoor pipe thermistor (R3T sensor) Common
The thermistor itself has drifted or failed completely. When tested with a multimeter, its resistance will read well outside the expected range for a 20 kΩ thermistor at room temperature. This is the most frequent root cause of a C5-00 alarm and is usually resolved by fitting a replacement sensor.
- Loose or corroded thermistor connector Sometimes
The connector linking the thermistor to the indoor PCB can work loose over time or develop corrosion — particularly in high-humidity indoor environments. A poor connection causes the PCB to receive an intermittent or out-of-range signal, triggering the fault even when the thermistor itself is electrically healthy.
- Faulty indoor unit PCB Rare
If the thermistor tests correctly and the connector is clean and secure, the fault can originate on the indoor PCB itself — either a failed input circuit on the sensor channel, or logic corruption caused by a power surge. PCB faults are less common but will produce an identical C5-00 alarm.
How to fix it
- Power-cycle the heat pump DIY safe
Turn the heat pump off at the controller, then switch off the isolator supplying the indoor unit. Leave it de-energised for at least five to ten minutes to allow the PCB to fully discharge, then restore power and restart. Occasional transient sensor faults caused by electrical noise can clear this way. If the C5-00 alarm returns immediately, proceed to the next steps.
- Check that the outdoor unit area is clear and system pressure is within range DIY safe
While the unit is off, take a moment to confirm there is no debris, ice or obstruction around the outdoor unit, and check that your system's filling-loop pressure gauge reads within the normal operating range (typically 1.0–2.0 bar when cold — consult your installer's handover documentation for your specific target). Neither of these will directly cause C5-00, but they are good practice before calling an engineer.
- Arrange for an engineer to inspect the thermistor connector heat pump engineer
A qualified heat pump engineer will open the indoor unit casing and visually inspect the thermistor connector for signs of looseness, corrosion or heat damage. Re-seating or cleaning the connector sometimes resolves the fault without any parts being required — a straightforward and inexpensive fix if caught early.
- Have the thermistor resistance tested and replaced if faulty heat pump engineer
The engineer will measure the resistance of the indoor pipe thermistor and compare it against the expected characteristic curve for a 20 kΩ sensor. If the reading is out of range, the thermistor will need to be replaced with a genuine compatible part. This is the most common outcome for a C5-00 fault.
- Replace the indoor PCB if the thermistor tests correctly heat pump engineer
Should the thermistor measure within specification and the connector be in good condition, the fault lies on the indoor PCB itself. The engineer will source a replacement board compatible with your indoor unit variant and carry out the swap, re-commissioning the unit afterwards.
- Contact an MCS-certified heat pump engineer to diagnose and repair the fault heat pump engineer
If you have not already done so, book an MCS-certified heat pump engineer to carry out steps 3–5. Daikin heat pumps should be serviced by a qualified HVAC professional to protect both your warranty and any remaining MCS installation guarantee. If refrigerant circuit work is ever required on the same visit, ensure the engineer also holds F-Gas registration.
Parts you may need
- Daikin indoor unit pipe thermistor (R3T, 20 kΩ) · from £25
- Daikin indoor unit PCB (model-specific) · from £320
- Thermistor connector harness/loom · from £18
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–£380, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using my heat pump while the C5-00 fault is showing?
The heat pump will typically suspend normal operation when C5-00 is active, so it is unlikely to be providing full heating or hot water. The fault itself is not dangerous, but leaving the system unresolved for more than a few days means your home will lose heating and hot water output. Book an engineer promptly rather than waiting and hoping it clears on its own.
How much does it cost to fix a Daikin C5-00 fault in the UK?
Most homeowners pay between £180 and £380 for a thermistor swap including parts and labour. If the indoor PCB turns out to be the culprit, costs typically rise to £450–£900 depending on the board and your location. Callout and diagnostic fees — usually £80–£150 — are often included within the total repair quote, so ask your engineer upfront.
Will my Daikin warranty cover a C5-00 thermistor fault?
Daikin's standard residential warranty covers manufacturing defects on parts for three years (and longer on some registered installations), but you must have had the unit serviced annually by a qualified engineer. If your service records are up to date and the thermistor has failed prematurely due to a manufacturing issue, your warranty claim has a strong basis. Wear-and-tear is excluded, and Daikin may request service history before approving any claim.
Can I replace the thermistor myself to save money?
The thermistor itself is a relatively simple component, but accessing it requires removing the indoor unit casing and working around live electrical wiring and control boards. We strongly recommend leaving this to a qualified heat pump engineer. Attempting it without the right training risks damaging the PCB, voiding your warranty, or creating a safety hazard. The parts cost is low; it is the labour and expertise you are paying for.