Daikin A1-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin A1-00 fault code mean?
The A1-00 code (also logged simply as A1) indicates that the indoor unit's main control board — sometimes called the PCB or 'brain' of the heat pump — has detected a fault within its own microprocessor or circuitry. The '00' suffix is the sub-code shown during self-diagnosis mode and simply confirms there is no additional sub-fault beyond the primary PCB error. When this fault is active, the indoor unit typically stops operating entirely, as the control board can no longer manage or co-ordinate the system's functions.
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
- Power surge or voltage spike Common
A sudden surge in mains voltage — caused by a lightning strike nearby, a grid event, or the system being powered back on after a prolonged outage — can damage the microprocessor or supporting components on the indoor PCB, triggering an A1 fault on restart.
- PCB component failure over time Common
Electronic components such as capacitors, MOSFETs and voltage regulators degrade with age and heat cycling. As these components fail, the PCB's microprocessor loses stable power or signal, causing it to log an A1 error. This is particularly common on units over seven years old.
- Manufacturing defect or early-life board failure Sometimes
In some cases, a latent defect in the PCB assembly itself — a dry solder joint, a substandard component, or a microscopic track fault — goes undetected at the factory and manifests within the first few years of operation. Daikin's own service documentation identifies board defects as a primary cause of A1.
- Electrical noise from external sources Sometimes
Interference from nearby machinery, variable-speed drives, or poorly earthed wiring can introduce electrical noise onto the signal lines of the indoor PCB, causing the microprocessor to lock up or report a fault. The A1 code may appear intermittently before becoming permanent.
- Connected sensor or wiring harness fault Rare
Occasionally, a faulty temperature sensor or a chafed wiring loom connected to the indoor PCB can cause signal errors that the board misinterprets as an internal failure, producing an A1 code even when the board itself is intact.
How to fix it
- Switch the heat pump off at the indoor isolator — not just the controller or remote — and leave it fully de-energised for at least five minutes. DIY safe
A brief, one-off power reset clears transient electrical faults that can occasionally trigger an A1 code. Use the dedicated isolator for the indoor unit rather than simply pressing standby on the wall controller. Five minutes is enough for capacitors to fully discharge and the microprocessor to reset cleanly.
- After five minutes, restore power at the isolator and observe the controller display for one to two minutes. DIY safe
If the unit starts normally and no fault code reappears, monitor it over the next 24 hours. If A1 returns immediately or within a short time, do not attempt further resets — repeated cycling will not resolve a genuine PCB fault and may obscure the fault history an engineer needs for diagnosis.
- Do not continue running the unit if the A1 code returns after the reset. DIY safe
With a control board fault active, the heat pump cannot safely manage its own components. Running it in this state risks additional electrical damage to interconnected parts. Switch it off at the isolator and leave it off until an engineer has inspected it.
- Check your warranty status before booking a repair. DIY safe
Daikin Altherma heat pumps installed by a Daikin D1 or D2 registered installer carry a five-year parts and labour warranty. If your unit is within warranty and has been serviced annually, a PCB replacement may be fully covered. Contact Daikin UK to check before paying for an independent repair call-out.
- Arrange a visit from an MCS-certified heat pump engineer for PCB diagnosis and replacement. heat pump engineer
The indoor control board must be inspected and tested with specialist diagnostic equipment. The engineer will confirm whether the board itself has failed or whether a connected sensor or wiring fault is responsible. If the PCB is confirmed faulty it will need to be replaced — this is not a repair suitable for a homeowner or a general electrician unfamiliar with heat pump systems.
Parts you may need
- Daikin indoor unit main PCB (control board) · from £320
- Indoor unit wiring harness / loom · from £45
- Indoor thermistor / temperature sensor set · from £35
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 £450–£900, depending on the underlying cause.
Frequently asked questions
Can I fix a Daikin A1-00 fault myself?
The only safe DIY action is a single power reset — switching the isolator off for five minutes and back on. If the A1 code clears and stays away, the fault was a temporary electrical glitch. If it returns, the PCB or a connected component has genuinely failed and you will need a qualified engineer. Replacing or repairing a heat pump control board involves working inside live electrical enclosures and requires specialist diagnostic tools; it is not a suitable homeowner repair.
How much does it cost to fix a Daikin A1-00 error in the UK?
Most homeowners pay between £450 and £900 all-in for a PCB diagnosis and replacement, including parts and labour. The spread depends on your specific indoor unit model, whether a Daikin-genuine or compatible board is used, and regional engineer rates. If your unit is within its five-year Daikin warranty, the replacement may be covered at no cost — it is worth checking before booking a private repair. On older or larger systems, if multiple boards or components are involved, costs can exceed £900, but that is uncommon for a straightforward A1 fault.
My Daikin heat pump shows A1-00 but worked fine after I reset it — should I be worried?
A single fault that clears after a reset and does not return is often caused by a brief power disturbance or electrical noise rather than a failing board. Keep an eye on the unit over the following weeks. If the A1 code appears again — even intermittently — book an engineer visit promptly. Intermittent PCB faults tend to worsen over time, and catching the problem early can sometimes mean a less expensive repair than waiting for a complete board failure.
Is it worth repairing a Daikin heat pump with an A1-00 fault, or should I replace it?
For a unit under ten years old in otherwise good condition, a PCB replacement is almost always worth doing — the rest of the system (compressor, refrigerant circuit, heat exchanger) will likely have many years of service left. On older units — particularly those already outside warranty, showing other recurring faults, or with high annual running costs — it may be worth getting a replacement quote at the same time as the repair quote, so you can compare. An MCS-certified engineer can give you an honest view on the overall condition of the system.