Daikin C4-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin C4-00 fault code mean?
The C4-00 fault code indicates that the indoor heat exchanger thermistor — the sensor responsible for monitoring the temperature of the indoor coil — is returning abnormal readings. The control board interprets this as either an open circuit (sensor disconnected or broken) or a short circuit (sensor or wiring shorted to earth). Because the heat pump relies on this reading to manage refrigerant superheat and subcooling safely, it enters a protective lockout rather than risk damage to the compressor or refrigerant circuit.
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 degraded indoor heat exchanger thermistor Common
The thermistor itself is the most frequent culprit. Over time the resistive element inside the sensor can drift out of specification or fail outright, producing either an infinite (open-circuit) or zero-ohm (short-circuit) reading. A healthy Daikin liquid-pipe thermistor typically measures around 20 kΩ at 25 °C; a reading far outside that range confirms sensor failure.
- Loose, corroded or damaged connector Common
The thermistor plugs into a connector on the indoor PCB. Vibration, heat cycling or simply a poorly seated plug can cause intermittent contact that triggers the fault. Visually the connector may look fine, but a slight tug or gentle wiggle during diagnosis is often enough to reveal a poor seating.
- Damaged wiring harness Sometimes
If the sensor lead has been kinked, pinched during a service visit, or partly chewed through, the conductor inside can break or short against the screening. The fault may appear intermittently at first before becoming permanent.
- Water ingress onto the sensor or PCB Sometimes
An internal water leak — from a poorly fitted joint or a blocked condensate drain — can deposit moisture onto the sensor head or its connector, causing a false short-circuit reading. Look for watermarks or corrosion near the sensor location.
- Low refrigerant charge Sometimes
An undercharged refrigerant circuit can produce wildly fluctuating coil temperatures that push the thermistor reading outside the control board's acceptance window. This is more likely if the C4 fault appears alongside other performance complaints such as poor heating output.
- Faulty indoor PCB Rare
If the thermistor and its wiring test within specification, the analogue input circuit on the indoor control board may have failed. PCB faults are less common but should be suspected once all other causes have been ruled out.
How to fix it
- Check for any obvious causes near the outdoor unit DIY safe
Before calling an engineer, do a quick external check. Clear any debris, leaves or compacted snow from around the outdoor unit that might be causing airflow problems contributing to abnormal sensor readings. Ensure the unit has not been physically disturbed.
- Inspect the system pressure DIY safe
Check the pressure gauge on the indoor unit or buffer vessel. Most Daikin heat pump systems should read between 1.0 and 2.0 bar when cold. If pressure is low, top up via the filling loop according to your system manual. Low pressure alone will not cause C4-00, but it confirms the system is being maintained and rules out compounding issues.
- Perform a hard power reset DIY safe
Turn the heat pump off at the controller, then switch off the dedicated circuit breaker (MCB) in the consumer unit. Leave the system fully de-energised for at least three minutes. Restore power and allow the unit to restart normally. A genuine temporary glitch can occasionally be cleared this way. If C4-00 reappears within a short time, the fault is real and requires a professional visit.
- Record the full fault code and any accompanying codes DIY safe
On most Daikin controllers, pressing and holding the 'Cancel' or 'Mode' button for around five seconds will display the stored fault history. Note down C4-00 and any secondary codes shown — this information will help the attending engineer diagnose the fault more quickly and may save you diagnostic time costs.
- Arrange an MCS-certified Daikin-approved engineer to inspect the thermistor and wiring heat pump engineer
The engineer will access the indoor unit, unplug the thermistor connector and test the sensor's resistance against the manufacturer's temperature-resistance curve. They will also check the wiring harness for continuity and inspect the connector for corrosion or damage. This is not a task for an unqualified person — incorrect handling of internal electrical components can cause further damage and void the warranty.
- Replace the thermistor with a genuine Daikin part if the sensor is confirmed faulty heat pump engineer
Daikin thermistors have a specific resistance curve; fitting a generic replacement with a slightly different curve can cause the unit to operate outside safe parameters, leading to poor performance or repeat faults. The engineer will seat the new sensor fully into its copper pocket and use thermal compound to ensure accurate thermal contact.
- Investigate refrigerant charge if the fault persists after sensor replacement heat pump engineer
If C4-00 returns or coil temperatures remain erratic after a confirmed good sensor is fitted, the refrigerant circuit should be checked for correct charge. Refrigerant work must be carried out by an F-Gas registered engineer — it is illegal for unqualified individuals to handle refrigerants in the UK.
- Replace the indoor PCB if all other components test satisfactorily heat pump engineer
When the thermistor, wiring and refrigerant charge are all confirmed good but the fault persists, the indoor control board is the remaining suspect. PCB replacement should be carried out by the attending engineer using a genuine Daikin board, as incorrect fitting or an incompatible board can cause further control faults.
- Contact an MCS-certified heat pump engineer if the fault has not self-cleared after a reset heat pump engineer
If the power reset in step 3 did not clear the fault, do not continue attempting workarounds. Book a visit from an MCS-certified, Daikin-approved engineer. For any work involving the refrigerant circuit, confirm the engineer also holds F-Gas registration.
Parts you may need
- Daikin indoor heat exchanger thermistor (liquid pipe sensor) · from £35
- Thermistor connector and wiring harness assembly · from £25
- Daikin indoor unit PCB (model-dependent) · from £280
- Thermal compound (for sensor pocket) · from £8
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–£380, depending on the underlying cause.
Frequently asked questions
Can I reset a Daikin C4-00 fault myself?
Yes, a power reset is worth attempting first and is safe to do yourself. Switch the unit off at the controller, turn off its dedicated MCB in the consumer unit, wait three minutes, then restore power. If C4-00 clears and does not return, the fault may have been a temporary glitch. However, if the code reappears — even after a successful reset — there is an underlying hardware problem that needs a professional diagnosis. Do not keep resetting without investigating further, as continued operation on a faulty sensor can stress the compressor.
How much does it cost to fix a Daikin C4-00 fault in the UK?
For most households, the repair falls between £150 and £380, which typically covers the engineer callout, diagnosis and thermistor or connector replacement with a genuine Daikin part. If the indoor PCB turns out to be at fault, costs usually rise to the £300–£600 range for the board plus labour. In rare cases where a refrigerant recharge is also needed, add the F-Gas engineer's time and refrigerant costs on top. Big-ticket components like a replacement compressor are very unlikely to be the cause of a C4-00 code specifically, but if a full compressor replacement were ever needed for a separate reason, that can run to £1,000–£2,000 or more.
Will my Daikin warranty cover the C4-00 fault?
If your heat pump is within its manufacturer or installer warranty period, the repair may be fully covered — contact Daikin UK or your installer in the first instance. Be aware that most Daikin warranties require evidence of annual servicing; if you cannot produce service records, Daikin may decline the claim. Daikin's own Gold maintenance package includes an annual service and breakdown cover, which can make warranty compliance easier to manage.
Do I need an F-Gas engineer to fix C4-00, or will any heat pump engineer do?
The C4-00 fault is primarily a sensor and wiring issue, so in most cases the repair does not touch the sealed refrigerant circuit. An MCS-certified heat pump engineer — ideally Daikin-approved — is the right person to call. However, if the diagnosis reveals a low refrigerant charge as a contributing factor, that portion of the work must be carried out by someone who also holds F-Gas registration, as handling refrigerants without certification is illegal in the UK. Always ask the engineer about both qualifications before booking.