Daikin 80-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 80-00 fault code mean?
The 80-00 code indicates that the Daikin Altherma heat pump has detected a problem with the R4T thermistor — the sensor responsible for measuring the temperature of water entering the heat exchanger inside the hydro module. Without a reliable reading from this sensor, the unit cannot regulate heat transfer safely and will shut itself down to prevent damage.
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
- Loose or corroded thermistor connector Common
The wiring plug connecting the R4T thermistor to the hydro PCB (A1P) can work loose over time due to vibration, or develop corrosion on the contacts — particularly in damp plantroom environments. This is the single most common trigger for the 80-00 code and is often fixable without replacing any components.
- Failed R4T thermistor Common
The thermistor element itself can develop an open or short circuit, causing its resistance to fall outside the expected range for the measured water temperature. An NTC thermistor that reads implausibly high or low resistance at a known water temperature has failed and needs replacing.
- Transient power disturbance Sometimes
A voltage spike, brownout, or brief mains interruption can temporarily scramble the signal from the thermistor to the hydro PCB, triggering the fault without any underlying hardware failure. A single power cycle is often enough to clear this.
- Faulty hydro PCB (A1P) Rare
If the thermistor and its wiring both check out correctly but the fault persists, the hydro module's printed circuit board may have lost the ability to correctly interpret the sensor signal. PCB failure is less common but should be suspected when multiple unrelated faults appear together.
How to fix it
- Switch off the heat pump and isolate it at the mains, then wait 10 minutes before restoring power and restarting from the controller. DIY safe
A transient fault caused by a power blip or brief disturbance can clear with a full power cycle. If the 80-00 code does not return within the first heating cycle, monitor the unit over the next 24 hours. If the code reappears, move to the next steps — repeated resets without further investigation are not recommended.
- Check that the outdoor unit and the area around the indoor hydro module are free from any obvious damage, and confirm the system pressure is within the normal operating range (typically 1.0–2.0 bar when cold). DIY safe
Low system pressure can cause the unit to behave erratically and trigger sensor-related lockouts. Top up via the filling loop if pressure has dropped below 1.0 bar. Also make sure no circuit breakers (MCBs) related to the heat pump have tripped at the consumer unit.
- Contact an MCS-certified heat pump engineer to inspect the R4T thermistor connector and wiring harness inside the hydro module. heat pump engineer
The engineer will check the wiring plug seating the R4T thermistor into the hydro PCB A1P for signs of looseness, corrosion, or damage. A reseated or cleaned connector often resolves the fault at minimal cost. This work requires opening the hydro module casing and should not be attempted by a homeowner.
- Have the engineer test the R4T thermistor resistance against the manufacturer's reference values. heat pump engineer
Using a multimeter, the engineer will measure the thermistor's resistance at the current water temperature and compare it to Daikin's expected resistance curve for an NTC sensor. A reading that is open-circuit, short-circuit, or significantly out of range confirms the thermistor has failed and needs to be replaced with the correct Daikin-approved part for your specific model.
- If the thermistor and wiring are found to be sound, have the engineer assess the hydro PCB A1P. heat pump engineer
Where the sensor itself is functioning correctly but the fault code persists, the hydro PCB is the next suspect. The engineer will check whether the PCB is receiving the correct supply voltage and whether it is processing the thermistor signal properly. PCB replacement is a more involved and costly repair, so confirming the thermistor is genuinely healthy first is important before committing to this.
- Ensure the correct replacement part is fitted and the fault code is cleared before the engineer leaves. heat pump engineer
Daikin thermistors are model-specific — an incorrect part number can re-trigger the same fault code. Ask the engineer to confirm the part reference against your unit's serial number and to carry out a test run to verify the heat pump completes a full heating cycle without the 80-00 code returning.
Parts you may need
- Daikin R4T Entering Water Thermistor · from £45
- Hydro PCB A1P (model-specific) · from £320
- Thermistor Wiring Harness / Connector Plug · from £25
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
Can I clear the Daikin 80-00 fault myself?
The only homeowner-safe step is a full power cycle — switching the unit off at the isolator, waiting 10 minutes, and restarting. If the code clears and does not return, a transient power disturbance was likely the cause. If it comes back, the fault requires a qualified engineer to inspect the thermistor and its wiring inside the hydro module. Do not repeatedly reset the unit hoping the fault will go away on its own.
How much does it cost to fix a Daikin 80-00 fault in the UK?
If the problem is a loose connector or a failed R4T thermistor, most homeowners pay in the region of £180–£380 including parts and labour. If the hydro PCB A1P is also found to be faulty, the repair cost rises considerably — PCB replacement typically adds £450–£900 to the bill. It is worth checking whether your unit is still within Daikin's warranty period, which covers manufacturer defects for up to five years on residential installations provided annual servicing has been carried out by a qualified engineer.
Will my Daikin Altherma keep heating my home while showing fault code 80-00?
No. The 80-00 code causes the heat pump to lock out and stop operating entirely. The unit cannot safely regulate heat exchange without a valid entering water temperature reading, so it shuts down as a protective measure. You will lose both space heating and hot water until the fault is resolved. If this happens during cold weather, arrange an engineer callout promptly rather than waiting.
Is the 80-00 fault related to the refrigerant circuit?
No — the 80-00 fault is entirely on the water side of the system. The R4T thermistor sits within the hydro module and monitors the temperature of water entering the plate heat exchanger, not refrigerant temperatures. Refrigerant-side sensor faults appear under different codes (82 and 83 on Daikin Altherma systems). This means the 80-00 repair does not require F-Gas refrigerant handling and is typically a straightforward sensor or PCB job for a heat pump engineer.