Daikin AA-02 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AA-02 fault code mean?
The AA-02 code on a Daikin Altherma heat pump indicates that the backup (booster) heater has exceeded its safe operating temperature, causing an internal safety device to trip. This is a protective shut-down — the system is telling you the electric backup heater inside the hydronic unit has overheated and needs attention before normal operation can resume.
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
- Blown thermal fuse or tripped thermal protector Common
The backup heater assembly contains a one-shot thermal fuse and a resettable thermal protector (Q1L). If the heater has overheated — even briefly — either or both of these safety devices may have activated. A blown thermal fuse will not reset and the heater element assembly must be replaced by an engineer.
- Air lock in the water circuit around the heater Common
If air has become trapped in the section of pipework surrounding the backup heater, water flow is restricted. Without sufficient flow to carry heat away, the heater overheats rapidly and triggers the AA-02 fault. This is a particularly common cause after system maintenance, filling or refilling.
- Faulty backup heater relay (K1M or K2M) Sometimes
The relays that switch the backup heater on and off can stick in the closed position, meaning the heater runs continuously even when it should not. Sustained operation at full load causes the heater to overheat and trip the safety cut-out.
- Faulty outlet thermistor on the backup heater Sometimes
A failed or out-of-calibration temperature sensor on the heater outlet can give the control board incorrect readings, preventing it from switching the heater off at the right time. The heater then runs beyond safe limits, triggering the fault.
- Faulty hydro PCB (A1P) or extension PCB (A4P) Sometimes
The hydronic circuit control board or the extension PCB manages heater operation, safety interlocks and sensor readings. A fault on either board can cause erratic heater control or prevent the safety response from operating correctly, resulting in an AA-02 error.
- Blown control fuse (FU2) Sometimes
A power disturbance, short circuit or wiring fault can blow the FU2 fuse on the PCB, disrupting control of the backup heater circuit and triggering the fault code.
- Incorrect system parameter settings Rare
If parameters such as the maximum tank temperature or backup heater enable threshold (e.g. parameter 4-00) have been set incorrectly — perhaps after a system configuration change or DHW schedule adjustment — the heater may be commanded to run at temperatures that cause it to overheat.
How to fix it
- Switch the heat pump off at the indoor controller and wait 5–10 minutes DIY safe
A full power-down allows the backup heater to cool and gives the safety devices a chance to reset if they are of the resettable type. Use the system controller or room thermostat interface to turn the unit off — do not simply flick the isolator at this stage, as a controlled shutdown is preferable.
- Check and restore system water pressure if needed DIY safe
Low system pressure can restrict water flow through the heater, contributing to overheating. Check the pressure gauge on the hydronic unit — it should typically read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top up via the filling loop until it reaches the correct level, then re-check for leaks. Consult your system handbook for the exact target pressure.
- Inspect the outdoor unit and indoor hydronic unit for obvious obstructions DIY safe
Clear any debris, leaves or accumulated snow from around the outdoor unit to ensure adequate airflow. Check that no valves on the indoor pipework circuit have been accidentally closed, which could restrict water flow to the heater.
- Restart the unit and observe whether the fault clears DIY safe
After the cool-down period, restore power and attempt a restart from the controller. If the AA-02 code does not reappear within the first heating cycle, monitor the system closely over the next 24 hours. If the fault returns, do not continue resetting — note how quickly the code reappears and report this to your engineer, as it helps diagnosis.
- Do not attempt to open the hydronic unit casing or inspect internal components heat pump engineer
The backup heater assembly, relays, PCBs, thermistors and fuses are all located inside the sealed hydronic unit enclosure. Working on these components requires isolation of the mains supply, specialist knowledge of the Daikin wiring schematics and, for any refrigerant-adjacent work, F-Gas registration. Attempting DIY access risks electric shock, voiding your warranty and potentially breaching UK regulations.
- Contact an MCS-certified heat pump engineer to carry out a full diagnosis heat pump engineer
A qualified engineer will check the thermal fuse and thermal protector (Q1L) in the heater assembly, test the backup heater relays (K1M and K2M), inspect the FU2 fuse, verify thermistor R5T readings, check the hydro PCB (A1P) and extension PCB (A4P), bleed any air from the water circuit and review system parameters such as the maximum tank temperature and backup heater settings. If the thermal fuse has blown, the heater element assembly will need replacement — this cannot be reset and must be done by a professional.
Parts you may need
- Backup heater element and thermal fuse assembly · from £180
- Thermal protector Q1L · from £35
- Backup heater outlet thermistor (R5T) · from £45
- Backup heater relay (K1M / K2M) · from £25
- Control fuse FU2 · from £10
- Hydro PCB A1P · from £320
- Extension PCB A4P · from £260
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–£550, depending on the underlying cause.
Frequently asked questions
Can I reset the Daikin AA-02 fault myself and carry on using the heating?
You can attempt a single power-cycle reset — switch the unit off at the controller, wait 10 minutes, then restart. If the fault clears and does not return, monitor the system carefully. However, if the code reappears, do not keep resetting it. The AA-02 indicates a genuine overheating event in the backup heater. Repeatedly resetting without fixing the underlying cause risks permanently damaging the heater assembly or, in extreme cases, causing a more serious electrical fault. Book an engineer as soon as possible.
Will the AA-02 fault affect my hot water as well as my heating?
Yes, in most Daikin Altherma configurations the backup heater contributes to both space heating and domestic hot water (DHW) production — particularly for DHW top-up in colder weather. While the fault is active and the backup heater is locked out, your heat pump compressor may still provide some heating, but hot water performance is likely to be reduced, especially in cold weather when the backup heater is needed most.
How much does it typically cost to fix the Daikin AA-02 fault in the UK?
Most homeowners with this fault pay between £180 and £550 all-in, depending on the failed component. A thermistor or relay swap sits at the lower end, while a full backup heater element and thermal fuse assembly replacement typically falls in the £300–£500 range including labour. A faulty PCB is less common but costs more — typically £450–£750 for parts and labour combined. If you are in doubt, ask your engineer for a written quote before authorising work.
Does a Daikin AA-02 fault void my warranty?
The fault itself does not void your warranty — it is a manufacturer safety feature working as intended. However, if an unqualified person has opened the hydronic unit and tampered with internal components, Daikin may reject a warranty claim. To keep your warranty valid, ensure all service work is carried out by an MCS-certified engineer and that your unit has been serviced annually. Keep your service records, as Daikin requires evidence of annual servicing to honour warranty claims.