Daikin AA-01 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin AA-01 fault code mean?
The AA-01 code appears on Daikin Altherma heat pumps and can carry one of several related meanings depending on the specific model in your installation. It most commonly signals a backup heater safety trip or overheat condition (triggered by the internal 26WH safety device), but on certain models it also indicates a power reset requirement following a communication disruption between the outdoor unit and the indoor control board. On EHBX-series indoor units in particular, AA-01 can point to a fault with the 3-way valve or an associated flow issue. Because the AA prefix is used across Daikin's Altherma range with slightly different sub-code definitions, always cross-reference with the service manual for your exact model.
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
- Temporary communication glitch or voltage fluctuation Common
A brief voltage sag, power interruption, or transient communication error between the indoor controller and the outdoor unit PCB can trigger AA-01 without any underlying hardware fault. In many cases a controlled power cycle resolves it entirely.
- Loose or corroded wiring connections Common
Over time, wiring terminations at the control board, sensor connectors, or the inter-unit communication cable can loosen or suffer corrosion. This causes intermittent signal loss that the system logs as a communication or control fault.
- Backup heater relay failure (K1M / K2M) Sometimes
The backup or booster heater relies on switching relays to operate safely. If either the K1M or K2M relay is worn, sticking or receiving incorrect parameter signals, the safety circuit trips and AA-01 is raised. An incorrect configuration in the heater kit or solar pump station settings can produce the same symptom.
- 3-way valve actuator fault (EHBX models) Sometimes
On EHBX indoor units, the 3-way valve directs flow between domestic hot water and space heating circuits. If the actuator becomes mechanically stuck, fails electrically, or a flow sensor connected to it develops a wiring fault, the system cannot verify correct circuit selection and raises AA-01.
- Outdoor unit PCB or temperature sensor fault Rare
A defective sensor or damaged outdoor PCB can send corrupted data back to the indoor controller, causing a communication breakdown that is logged as AA-01. This is less common but typically requires component-level diagnosis.
How to fix it
- Switch the unit off at the mains isolator and leave it fully de-energised for at least 5 minutes DIY safe
Locate the dedicated isolator switch for the heat pump (usually adjacent to the indoor unit or consumer unit). Turn it off and wait a full 5 minutes to allow capacitors and control boards to fully discharge and reset. Then switch the isolator back on and allow the system 10 minutes to restart its initialisation sequence before checking whether the fault has cleared.
- Check the consumer unit for any tripped MCBs associated with the heat pump circuit DIY safe
Open your consumer unit and confirm that the circuit breaker(s) labelled for the heat pump or cylinder heater are in the ON position. If one has tripped, reset it once. If it trips again immediately, do not keep resetting it — call an engineer, as repeated tripping indicates a wiring or component fault downstream.
- Inspect the outdoor unit for any physical obstructions DIY safe
Walk around the outdoor unit and clear away any leaves, debris, or accumulated ice that may be blocking the air inlet or outlet grilles. Ensure there is at least 30–50 cm of clear space around the unit. Restricted airflow can contribute to overtemperature conditions that trigger safety devices.
- Check system water pressure on the indoor unit gauge DIY safe
Locate the pressure gauge on the indoor unit or buffer vessel. For most Daikin Altherma systems, normal operating pressure when cold is between 1.0 and 1.5 bar. If it reads below 0.8 bar, top up via the filling loop (usually a braided hose with a double-handled valve beneath the indoor unit) until the gauge returns to the correct range, then close the loop securely.
- If the fault clears, monitor the system closely over the next 24–48 hours DIY safe
A fault that does not return after a power reset is likely to have been caused by a transient voltage event. Make a note of the date and time it occurred. If the code reappears within a short period, do not continue resetting — this masks the underlying issue and may delay a diagnosis while the system operates in a degraded state.
- Arrange for an MCS-certified heat pump engineer to inspect wiring, connections and the backup heater relay heat pump engineer
If the fault persists or returns, a qualified engineer should perform a full inspection of the inter-unit communication cabling, wiring terminations at both the indoor and outdoor PCBs, and the condition of the backup heater relays (K1M and K2M). Wiring faults and relay failures require safe isolation and component-level testing that must not be attempted by a homeowner.
- Have the engineer inspect the 3-way valve actuator and associated flow sensors (EHBX models) heat pump engineer
On EHBX indoor units, the engineer should verify that the 3-way valve is moving through its full range of travel, that the actuator is receiving the correct control signal, and that any flow sensors connected to the valve circuit are reading accurately. Mechanical replacement or electrical repair of the actuator is a workshop task, not a homeowner task.
- Request a full backup heater kit check and parameter review heat pump engineer
The Daikin Altherma service manual specifically directs technicians to verify the backup heater kit configuration when the AA error is raised. The engineer should confirm that all heater kit parameters are correctly set for your installation, particularly if a solar pump station or additional heat source is integrated into the system.
Parts you may need
- Backup heater relay (K1M / K2M) · from £55
- 3-way valve actuator · from £120
- Inter-unit communication cable · from £35
- Outdoor unit main PCB · from £320
- Flow sensor / water temperature sensor · from £45
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–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Daikin AA-01 myself with a simple reset?
A power cycle (isolator off for 5 minutes, then back on) is the one homeowner-safe action you can try, and it does resolve AA-01 in cases where the cause was a brief voltage fluctuation or communication glitch. However, if the code returns after a reset — or comes back repeatedly over days or weeks — you should stop resetting and call an MCS-certified engineer. Repeated resets without addressing the root cause can mask a developing fault and may lead to more expensive damage over time.
Is my Daikin Altherma still under warranty?
Daikin Altherma systems installed by a Daikin D1 or D2 registered installer carry a 5-year parts and labour warranty, provided the warranty was registered at daikin.co.uk/warranty within 30 days of installation. If your system is within that period, contact Daikin UK's Residential Support team first, as a warranty repair will be arranged through their own engineer network at no cost to you. Check your installation paperwork or the Daikin website to confirm your registration status before paying for a third-party call-out.
How much does it typically cost to repair AA-01 if a part needs replacing?
Most AA-01 repairs in the UK fall in the range of £120–£450 once you include the engineer's diagnostic time and any parts replaced. A wiring repair or relay swap sits towards the lower end; a 3-way valve actuator replacement or flow sensor swap typically lands in the middle. If the fault turns out to be the outdoor PCB, costs can climb significantly higher — PCB replacements on Daikin Altherma units can reach £500–£700 or more including labour — but this is not the typical outcome for AA-01.
Does AA-01 mean my refrigerant circuit has a problem?
Not directly. AA-01 primarily relates to the backup heater circuit, communication between the indoor and outdoor units, or the 3-way valve — none of which are part of the sealed refrigerant circuit. However, if an engineer's diagnosis does identify a refrigerant-side issue as a contributing factor, that work must only be carried out by an F-Gas registered engineer, as handling refrigerant without the appropriate certification is illegal in the UK.