Daikin AA Fault Code: Heater Overheat / Sensor Activation Fault
The Daikin AA fault code carries two distinct meanings depending on your system type. On Daikin AC, VRV, and split systems it signals a heater overheat condition — specifically the activation of the 26WH internal safety device, which trips to protect the unit from excessive temperatures. On Daikin Altherma Hybrid air source heat pump systems the same code points to a sensor or activation fault that commonly results in a loss of hot water or heating output. Because the meaning differs across the product range, it is important to identify which type of Daikin system you have before troubleshooting.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Daikin AA fault code mean?
The Daikin AA fault code carries two distinct meanings depending on your system type. On Daikin AC, VRV, and split systems it signals a heater overheat condition — specifically the activation of the 26WH internal safety device, which trips to protect the unit from excessive temperatures. On Daikin Altherma Hybrid air source heat pump systems the same code points to a sensor or activation fault that commonly results in a loss of hot water or heating output. Because the meaning differs across the product range, it is important to identify which type of Daikin system you have before troubleshooting.
Common causes
- 26WH safety device activation (AC/split systems) Common
On conventional AC and VRV units the 26WH overheat thermostat trips when the heater exceeds its safe operating temperature. This is typically caused by restricted airflow around the unit, a component failure within the heater circuit, or a faulty overheat sensor that triggers prematurely. The boiler or heater cannot restart until the underlying cause is investigated.
- Sensor wiring damage or loose connections (Altherma Hybrid) Common
On Altherma Hybrid systems, wiring to temperature sensors can deteriorate over time — particularly in loft installations where cables may be exposed to pests or general wear. Corroded or loose connections at the control board or near the outdoor unit are a frequent culprit and can fool the system into registering a fault even when the hardware itself is sound.
- Temporary voltage fluctuation or communication error Sometimes
A brief dip in mains voltage or a momentary loss of communication between the indoor and outdoor units can trigger the AA code without any underlying mechanical fault. In these cases a full power reset is often sufficient to clear the fault and restore normal operation.
- Low refrigerant charge (Altherma Hybrid) Sometimes
A slow refrigerant leak in the heat pump circuit reduces system efficiency and can cause temperature-related fault codes including AA. Refrigerant loss is particularly common in hybrid systems that have been installed for several years or where pipework joints have not been properly pressure-tested. This must be investigated by an F-Gas certified engineer — it is illegal for unqualified individuals to handle refrigerants.
- Faulty outdoor unit PCB or temperature sensor Rare
If a power reset and wiring checks do not resolve the fault, the root cause may be a failed PCB or a defective temperature sensor on the outdoor unit. These components require specialist diagnosis and replacement by a qualified Daikin engineer.
How to fix it
- Check whether your system is within its Daikin warranty period DIY safe
Daikin Altherma systems installed by a Daikin D1 or D2 registered installer benefit from a 5-year parts and labour warranty, provided the warranty was registered at daikin.co.uk/warranty within 30 days of installation. If your unit may still be covered, contact Daikin UK directly before commissioning any paid repairs — this could save you several hundred pounds.
- Perform a full power reset DIY safe
Switch the unit off completely at the main isolator or consumer unit and leave it without power for at least 5 minutes. This allows any temporary communication fault or voltage glitch to clear. After 5 minutes, restore power and check whether the AA code reappears. Do not repeat this more than 2–3 times; if the fault returns consistently a reset alone will not fix the underlying problem.
- Inspect visible wiring for obvious damage DIY safe
With the unit powered down, visually examine any accessible wiring — particularly runs in loft spaces — for signs of rodent damage, chafing, or corrosion. Do not open the outdoor unit casing or touch internal components. If you spot damaged insulation or a loose plug connector at an accessible junction, note its location to report to your engineer. Do not attempt to repair wiring on a refrigerant-containing system yourself.
- Contact an F-Gas certified engineer for refrigerant and sensor diagnosis Gas Safe engineer
If the fault persists after a reset, or if you suspect a refrigerant leak (reduced heating output, ice forming on the outdoor unit), you must instruct an engineer who holds F-Gas certification. It is a legal requirement in the UK for anyone handling refrigerants to be F-Gas qualified. Note that Daikin Altherma systems are a specialism — not all heating engineers are familiar with them, so ask specifically for Daikin Altherma experience when booking.
- Have a Gas Safe registered engineer assess any gas-side components on Altherma Hybrid systems Gas Safe engineer
Daikin Altherma Hybrid units incorporate a gas boiler alongside the heat pump. Any work on the gas boiler element — including the gas valve, heat exchanger, or burner — must be carried out by a Gas Safe registered engineer. Ensure whoever you book holds both F-Gas certification and Gas Safe registration, or that you arrange separate qualified tradespeople for each part of the system.
Parts you may need
- 26WH overheat safety thermostat (AC/split systems) · from £45
- Outdoor unit temperature sensor (Altherma Hybrid) · from £55
- Outdoor unit PCB (Altherma Hybrid) · from £280
- Refrigerant recharge (R32 or R410A, labour and gas included) · from £450
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 £200–£600, depending on the underlying cause.
Frequently asked questions
Can I fix the Daikin AA fault code myself?
The only steps a homeowner can safely carry out are a full power reset and a visual inspection of accessible wiring. Everything else — including any work on the refrigerant circuit, sensors, PCB, or gas-side components — must be done by a qualified engineer. Handling refrigerants without F-Gas certification is illegal in the UK, and gas work requires Gas Safe registration.
How much does it cost to fix a Daikin AA fault in the UK?
Most people pay somewhere between £200 and £600 depending on what is found. A refrigerant recharge with leak investigation typically sits around £300–£500, while a specialist call-out for diagnosis alone can cost £150–£200 before any parts. If the outdoor unit PCB has failed, costs can climb above £600 once parts and labour are combined. If your system is within Daikin's 5-year installer warranty, repairs may be covered at no cost — worth checking before booking anyone.
Why does the Daikin AA code mean different things on different units?
Daikin uses a shared fault-code library across a very wide product range. On traditional AC and VRV systems, AA specifically refers to the 26WH heater overheat safety device. On Altherma Hybrid heat pump systems the same code is mapped to a sensor or activation fault in the heat pump circuit. Always check which type of system you have — the model sticker is usually on the indoor unit — and cross-reference with your model's specific documentation before drawing conclusions.
My Daikin Altherma shows AA and I have no hot water — is it safe to use?
You should not continue to run the system with a persistent AA fault. The code indicates the unit has detected a problem and in many cases the system will have locked itself out as a precaution. Using it in fault state risks causing further damage. Switch it off, attempt a single power reset, and if the fault returns call a qualified engineer promptly. In the meantime you may need to use an immersion heater if you have one, or arrange temporary alternative hot water.