Daikin 81-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 81-00 fault code mean?
The 81-00 code indicates that the heat pump's indoor hydronic unit has detected an abnormal reading from the leaving water thermistor (R1T) — the sensor responsible for measuring the temperature of water as it exits the heat pump. When the system receives an implausible or missing signal from this thermistor (caused by an open or short circuit in the sensor), it shuts down as a protective measure. The unit will attempt an automatic restart once the underlying issue clears, but if the root cause persists, the fault will return.
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
- Open or short circuit in the R1T thermistor probe Common
The leaving water thermistor R1T itself has failed internally — either its resistance has gone out of range (open circuit) or it has shorted. This sends an implausible temperature reading to the hydro PCB, triggering the fault. Thermistor probes can degrade over time, particularly if moisture has entered the sensor body or if there has been localised overheating.
- Loose, damaged, or corroded wiring connector Common
The connector joining the R1T thermistor to the hydro PCB (A1P) has become loose, corroded, or has a broken wire. Vibration from normal heat pump operation and seasonal temperature swings can work connectors loose gradually. Even a partially disconnected plug can cause the PCB to read the circuit as open and log the fault.
- Faulty hydro PCB (A1P) Rare
If the thermistor probe and all wiring connections check out as healthy, the fault may lie with the hydro PCB itself. The PCB is responsible for interpreting the thermistor's resistance signal and converting it to a temperature reading — if the relevant input circuit on the board has failed, a perfectly good thermistor will still produce a fault code.
How to fix it
- Restart the heat pump from the controller DIY safe
Switch the unit off at the controller, wait around two minutes, then switch it back on. The 81-00 fault includes an auto-reset feature, meaning if the sensor reading has recovered (for example, after a momentary connection drop), the unit may resume normal operation. Note the time and whether the fault reappears — this information will be useful for your engineer.
- Check the isolator switch and consumer unit MCB DIY safe
Make sure the heat pump's dedicated MCB in your consumer unit has not tripped, and that the outdoor and indoor isolator switches are both in the ON position. A partial power interruption can occasionally cause sensor faults to be logged.
- Do not attempt to access internal wiring or sensors yourself heat pump engineer
The R1T thermistor and its connector are located inside the indoor hydronic unit. Accessing these components requires removing panels from the unit and working near live electrical circuits and pressurised refrigerant pipework. This work must only be carried out by a qualified engineer.
- Have an engineer check and test the R1T thermistor and its wiring heat pump engineer
An MCS-certified heat pump engineer will use a multimeter to measure the thermistor's resistance and compare it against Daikin's published resistance-temperature curve for the R1T sensor. They will also inspect the connector for corrosion, pin damage, or loose seating. If the thermistor is found to be out of specification, it will be replaced.
- If the thermistor proves healthy, have the hydro PCB (A1P) inspected and replaced if necessary heat pump engineer
Should the R1T thermistor and all associated wiring pass inspection, the engineer will investigate the hydro PCB (A1P). A faulty input channel on the board can mimic the symptoms of a failed thermistor. PCB replacement is a more involved job and a more significant cost — your engineer will confirm the diagnosis before proceeding.
- Call an MCS-certified heat pump engineer if the fault persists after a reset heat pump engineer
If the 81-00 code returns after a restart, book a service visit with an MCS-certified heat pump engineer. Avoid repeatedly resetting the unit in the hope the fault will clear on its own, as this can obscure the pattern of the fault and delay diagnosis. Any work involving the refrigerant circuit must be carried out by an F-Gas registered engineer.
Parts you may need
- Leaving water thermistor R1T (Daikin Altherma) · from £35
- Thermistor connector/wiring harness · from £20
- Hydro PCB A1P (Daikin Altherma indoor unit) · 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–£380, depending on the underlying cause.
Frequently asked questions
Will my heating stop working completely when the 81-00 code appears?
Yes — the 81-00 fault causes the heat pump to stop operating as a protective measure, so you will lose both heating and hot water until the fault clears or is repaired. The unit does include an auto-reset function, meaning it may briefly recover if the sensor connection was intermittent, but if the underlying problem is a failed thermistor or PCB, the fault will return. If you have an immersion heater as a backup in your hot water cylinder, you can use that for hot water in the short term while you wait for an engineer.
Can I fix the 81-00 fault myself?
The initial reset (switching the unit off at the controller and back on) is something you can safely try yourself. However, the actual repair — testing the R1T thermistor, checking its connector, and replacing either the thermistor or the hydro PCB — requires opening the indoor hydronic unit and working near electrical components. This should only be done by an MCS-certified heat pump engineer. Attempting it yourself could void your warranty and may be unsafe.
How much does it cost to fix a Daikin 81-00 fault in the UK?
For the most common scenario — a failed R1T thermistor or loose connector — you can typically expect to pay in the region of £180 to £380 including the engineer's callout, parts, and labour. If the hydro PCB (A1P) turns out to be the cause rather than the thermistor, the cost rises significantly, with PCB supply and fitting often coming to £450–£900 depending on the specific model and the engineer's rates. Your engineer should diagnose the thermistor first, as that is by far the more likely and less expensive fault.
Does this fault affect my Daikin warranty?
Daikin's residential warranty typically covers manufacturer defects for three years from installation, provided the unit has been serviced annually by a suitably qualified engineer. If your heat pump is within its warranty period and you have up-to-date service records, the repair may be covered at no cost — contact Daikin or your installing MCS-certified engineer to check. Without evidence of annual servicing, Daikin may decline the warranty claim, so keeping service records is important.