Daikin AH-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AH-00 fault code mean?
The AH-00 code indicates that the heat pump's automatic tank disinfection cycle did not complete successfully. Daikin Altherma systems are programmed to periodically raise the hot water cylinder temperature to around 60°C and hold it there for a set duration in order to eliminate the risk of Legionella and other bacteria building up in stored water. When the system cannot reach that target temperature or cannot sustain it for the required period — whether due to a component issue or simply because hot water was drawn off mid-cycle — it logs AH-00 as a caution. Crucially, this is a warning-level code rather than a hard lockout: the heat pump typically continues to operate and will attempt to repeat the cycle automatically.
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
- Hot water drawn off during the disinfection cycle Common
If someone showers, runs a bath, or uses a tap while the disinfection programme is running, cold mains water entering the cylinder drops the stored temperature below the required threshold. The cycle then cannot complete within its allotted time and the unit logs AH-00. This is by far the most frequent trigger and usually clears on the next scheduled attempt.
- Disinfection or DHW temperature settings configured too low Common
If the target temperature for the disinfection function has been set below 60°C, or if the backup/immersion heater is disabled or restricted in the controller settings, the cylinder may never reach the required threshold regardless of demand. Checking the disinfection temperature parameter and the backup heater enable setting (e.g. parameter 4-00 on some Altherma models) is an important early step.
- Faulty or drifting tank thermistor (R5T) Sometimes
The cylinder thermistor R5T reports the stored water temperature to the hydro PCB. If it has developed a resistance fault or poor connection, the controller may believe the target temperature has not been reached even when it has — or vice versa — preventing the cycle from being signed off as complete.
- Backup or booster heater not operational Sometimes
The disinfection cycle often relies on the immersion/backup heater to top the cylinder temperature up to 60°C, especially in cooler weather when the heat pump alone may struggle. If the heater element has failed, its contactor has tripped, or it has been isolated at the fused spur, the cylinder cannot reach the target and the fault is logged.
- Hydro unit PCB fault (A1P or A4P) Rare
The main hydro PCB (A1P) and any extension PCB (A4P) coordinate the disinfection sequence. A board fault — whether from a power surge, moisture ingress or component failure — can prevent the cycle from being initiated or concluded correctly, causing a persistent AH-00 even when thermistors and heaters are working normally.
- Poor system water circulation during the cycle Sometimes
If the primary circulation pump is underperforming or the system pressure is low, heat may not be distributed evenly through the cylinder during the disinfection run. Localised cool spots can prevent the overall stored volume from reaching temperature uniformly, stalling the cycle.
How to fix it
- Allow the system to retry automatically DIY safe
Because AH-00 is a caution rather than a lockout, the Daikin Altherma will schedule another disinfection attempt without any intervention. If the code appeared because hot water was used mid-cycle, simply leave the unit running and avoid large draws of hot water during the next scheduled cycle window (often set to an overnight or early-morning slot). Check whether the code clears on its own within 24 hours.
- Reset the unit from the controller DIY safe
If the code persists after the automatic retry window, perform a soft reset from the Daikin controller or LAN adapter interface. Power the unit off using the main isolator, wait five minutes to allow capacitors to discharge, then restore power. This clears any temporary fault latch and allows the system to begin a fresh disinfection attempt.
- Check the outdoor unit and indoor hydro unit for obvious issues DIY safe
Walk around the outdoor unit and confirm there is no ice build-up, debris blockage or standing water around the base that might be causing the unit to operate inefficiently. Inside, check that the system pressure gauge on the hydro unit reads between 1.0 and 2.0 bar. If pressure is below 1.0 bar, top up via the filling loop in small increments until you reach approximately 1.5 bar, then re-check for any visible leaks.
- Review the disinfection and backup heater settings in the controller menu DIY safe
Access the installer or user menu on the Daikin controller and confirm that the disinfection target temperature is set to at least 60°C and that the backup or booster heater is enabled. On many Altherma models, parameter 4-00 or equivalent controls whether the electric heater is permitted to assist during the DHW cycle. If these settings have been inadvertently changed — perhaps after a power cut — restoring them to the recommended values may resolve the fault immediately. Consult your installer handbook for the specific parameter reference for your model.
- Check the error history log on the controller DIY safe
On most Daikin Altherma controllers, holding the relevant mode button for around five seconds will display a blink-coded error history. Note any additional codes stored alongside AH-00 — for example, a thermistor or sensor code — as this will help an engineer diagnose the root cause more quickly and reduce the time (and cost) of the visit.
- Verify the backup/immersion heater fused spur is switched on DIY safe
Locate the dedicated fused spur or isolator for the immersion/backup heater on your consumer unit or close to the cylinder. Confirm it has not been switched off or tripped. If the MCB for this circuit has tripped, reset it once. If it trips again immediately, do not force it — leave it off and report this to your engineer, as it indicates a wiring or element fault.
- Contact an MCS-certified heat pump engineer if the fault persists heat pump engineer
If AH-00 continues to appear after the checks above, an engineer will need to test thermistor R5T with a multimeter to verify its resistance curve, inspect wiring connectors at the hydro PCB for corrosion or looseness, and confirm that both the A1P and A4P PCBs are receiving correct supply voltages. Any work inside the electrical enclosure of the hydro unit or on control boards must be carried out by a suitably qualified engineer. If a refrigerant-circuit component is also implicated, the engineer must hold a valid F-Gas qualification.
Parts you may need
- Tank thermistor R5T (cylinder sensor) · from £35
- Backup/immersion heater element · from £65
- Hydro unit main PCB (A1P) · from £320
- Extension PCB A4P · from £180
- Circulation pump (hydro unit) · from £150
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Is the Daikin AH-00 code serious — will my heating stop working?
In most cases, no. AH-00 is a caution-level warning specifically related to the hot water disinfection cycle, and the heat pump typically keeps running space heating and normal hot water functions while it is displayed. The unit will attempt the disinfection cycle again automatically. If the code clears within a day or two, no repair is needed. Only if it appears repeatedly or is accompanied by other fault codes should you treat it as urgent.
Can I fix the Daikin AH-00 error myself, or do I need an engineer?
Several of the most common causes — a mid-cycle hot water draw, a tripped heater spur, or an incorrect controller setting — can be resolved by the homeowner without any special tools. Work through the checks listed above first. However, if the fault points to a failed thermistor, a PCB problem or anything inside the electrical enclosure of the hydro unit, you will need an MCS-certified engineer. Attempting internal electrical repairs yourself risks personal injury, may invalidate your warranty, and could affect your MCS certificate documentation.
How much will it cost to repair an AH-00 fault on a Daikin Altherma?
If the fix is a settings adjustment or a simple sensor reconnection, you are likely looking at a single callout charge — typically £120 to £250 depending on whether you use a Daikin-contracted engineer or an independent specialist. Replacing a faulty thermistor or a backup heater element will usually bring the total to £150–£400 including parts and labour. A main hydro PCB replacement is a less common outcome but can push costs higher — this is worth asking about if your system is out of warranty, as it may influence whether repair or replacement is the better long-term decision.
How do I stop the AH-00 code from coming back?
The most reliable preventative measure is to schedule the disinfection cycle for a time when no one in the household is likely to use hot water — a 2–4 am slot works well for most families. Also ensure your system receives an annual service from an MCS-certified engineer: they will check the thermistor calibration, confirm the backup heater is operating correctly, and verify that the disinfection parameters are set appropriately. Keeping a Daikin maintenance contract active not only helps prevent recurrence but also reduces callout costs if a fault does develop.