Daikin AJ-03 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AJ-03 fault code mean?
The AJ-03 warning appears on Daikin Altherma heat pumps when the domestic hot water (DHW) cylinder has taken longer than 6 hours to reach its target temperature. Unlike a hard lockout fault, the system keeps running — but the warning flags that something is preventing the hot water from heating up within a normal timeframe. This could be a settings issue, a component fault, or simply unusually high hot water demand in the household.
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
- Excessive hot water demand Common
If several people in the household have showered, run baths, or used large amounts of hot water in a short period, the cylinder may simply have been depleted faster than the heat pump can recover it. This is especially common in larger households or during cold weather when incoming mains water is colder than usual.
- DHW schedule too short or misconfigured Common
If the hot water heating window in the controller or the Daikin app is not long enough, or if there is a mismatch between the settings on the on-unit MMI and those in the app, the cylinder may not have enough time to reach temperature. A disinfection cycle in particular can fail to complete if the schedule is too tight.
- Backup or booster heater not enabled or faulty Common
The immersion-style backup or booster heater is often needed to top up the cylinder quickly, particularly in cold weather. If it has been disabled in the settings, has a blown fuse, or has developed a fault, the heat pump alone may struggle to hit the target temperature in time.
- Faulty or stuck 3-way valve Sometimes
The 3-way valve diverts the refrigerant circuit between space heating and DHW modes. If it is stuck, slow to switch, or not fully diverting flow to the DHW circuit, the cylinder will heat very slowly or not at all, triggering AJ-03.
- DHW tank thermistor (R5T) fault Sometimes
The R5T thermistor measures water temperature inside the cylinder. If it is reading incorrectly — either due to a wiring fault, poor contact, or a failed sensor — the system may think the cylinder is cooler than it actually is and keep running beyond the expected time limit.
- Leaking DHW pipework or hot water tap Sometimes
A dripping or continuously flowing hot water tap, or a leak in the DHW pipework, means cold water constantly enters the cylinder, making it almost impossible for the system to maintain temperature and causing excessively long heat-up times.
- PCB or power supply issue Rare
A fault on the hydro unit PCB (A1P or A4P), a blown onboard fuse, or an unstable power supply to the unit can disrupt how components such as the heater or 3-way valve are controlled, leading to the AJ-03 warning as a downstream symptom.
How to fix it
- Check the outdoor unit and surrounding area DIY safe
Walk around the outdoor unit and confirm it is free from obstructions — snow, ice build-up, leaves, or debris blocking the airflow will reduce efficiency significantly. Clear anything within at least half a metre of the unit on all sides.
- Check the system water pressure DIY safe
Look at the pressure gauge on the indoor unit or buffer vessel. It should read between 1.5 and 2 bar when cold. If it is sitting at or below 1 bar, the system is undercharged. Consult your installer's handover notes or the unit manual for how to top up pressure using the filling loop — this is a homeowner-safe task on most installations.
- Review your DHW schedule on both the MMI and the Daikin app DIY safe
Open the on-unit controller (MMI) and the Daikin app and compare the DHW heating schedules. Make sure the hot water heating window is long enough — typically at least 2–3 hours for a standard cylinder, longer in winter. Ensure the backup or booster heater is enabled in the settings if your system has one. Correct any discrepancies between the app and the MMI, as conflicting settings are a surprisingly common trigger.
- Consider recent hot water usage DIY safe
If several people have used hot water in quick succession, or an unusually large draw-off occurred (e.g. filling a bath), the warning may simply be a one-off event. Monitor whether it clears once the cylinder recovers and assess whether your cylinder size and settings are appropriate for your household's demand.
- Reset the unit at the isolator DIY safe
Turn the heat pump off at the red isolation switch (usually on the wall near the outdoor unit or indoor unit). Wait at least 5 minutes, then switch it back on and allow the system to restart. Monitor whether the fault returns. A one-off warning that does not reappear after a reset and a review of your schedule is unlikely to need an engineer visit.
- Check for dripping taps or leaks on the hot water pipework DIY safe
Check all hot water taps and any visible DHW pipework for drips or continuous flow. Even a slow drip can introduce enough cold water to prevent the cylinder from holding temperature. Fix any dripping taps or ask a plumber to do so.
- Have an MCS-certified engineer inspect the 3-way valve, heater elements, thermistor and PCBs heat pump engineer
If the warning persists after the homeowner checks above, the fault is likely inside the unit. A qualified heat pump engineer will need to test the 3-way valve operation, check the DHW thermistor (R5T) for accuracy, inspect the backup and booster heater elements and their fuses, and examine the hydro PCBs. None of these tasks are safe or appropriate for a homeowner to attempt. If your unit is still under warranty, contact Daikin before booking an independent engineer to ensure the visit is authorised and covered.
Parts you may need
- 3-way valve (DHW/space heating diverter) · from £120
- DHW tank thermistor R5T · from £35
- Backup immersion heater element · from £65
- Booster heater assembly · from £110
- Hydro PCB A1P · from £280
- Hydro PCB A4P · from £220
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 £150–£500, depending on the underlying cause.
Frequently asked questions
Is the AJ-03 fault serious — will my heat pump shut down?
No, AJ-03 is a warning rather than a lockout. Your Daikin Altherma will continue operating, but it is flagging that the hot water cylinder is taking too long to reach its target temperature. You should investigate the cause, but you are unlikely to be left with no heating or hot water immediately. That said, if the underlying cause is a faulty component such as the 3-way valve or a heater element, the problem will not fix itself and should be addressed promptly.
Can I fix AJ-03 myself, or do I need an engineer?
Several of the most common triggers — a too-short DHW schedule, a settings mismatch between the app and the MMI, low system pressure, or a temporary spike in hot water demand — are things a homeowner can check and correct without any specialist knowledge. Start by reviewing your schedule settings, checking system pressure, and doing a reset at the isolator. If the fault keeps coming back after those checks, you will need an MCS-certified heat pump engineer to investigate internal components such as the 3-way valve, heater elements, and thermistor.
How much will it cost to fix AJ-03 on my Daikin Altherma?
If the fix is a settings adjustment or a system reset, the only cost is an engineer call-out if you need one — typically £150–£300 depending on whether you use a Daikin-direct service or an independent MCS-certified engineer. For a component repair, a thermistor replacement typically comes to around £150–£250 all in, while a 3-way valve or heater element replacement usually falls in the £250–£500 range including labour. PCB replacements can cost more and are worth discussing with your engineer before authorising. If your unit is within its warranty period, contact Daikin before booking any work.
Why does AJ-03 keep coming back even after a reset?
A recurring AJ-03 warning after a reset almost always points to a component fault or a persistent settings problem rather than a one-off event. The most common culprits are a partially stuck 3-way valve that is not fully diverting flow to the cylinder, a backup or booster heater that has failed or been accidentally disabled, or a DHW schedule that is genuinely too short for your household's hot water demand. A misreading thermistor is another possibility. If you have already reviewed your schedule and confirmed settings are correct, book an MCS-certified engineer to carry out a thorough inspection.