What it means
The Altherma 3 R ECH2O guide words it as "Backup heater overheated or BUH power cable not connected". AA-02 is used when an external backup heater overheats. Daikin's service manual says AA-01 needs a reset at the controller and a manual reset of the backup heater's thermal protector.
What to check, in order
- Check the water pressure
Daikin's service manual lists low water pressure, low water flow and air in the water circuit among the causes. Check the pressure on the home screen is above 1 bar.
- Leave the cut-out to your installer
Daikin's service manual says the backup heater's thermal protector has to be reset by hand as well as at the controller. Leave that to your installer and don't open the unit yourself.
- Read the help text
On the controller, open the main menu and choose Malfunctioning to see the code and a short description. The help button gives a longer one. Note the code for your installer.
When to call your installer
Call your installer, who will find the cause before resetting it. Daikin's service manual lists low water pressure or flow, air in the water circuit, an external heat source, a faulty thermal protector, a faulty outlet water thermistor, a faulty backup heater or a faulty hydro PCB.
A repair visit will usually check the water flow, the valves and the temperature sensors.
- Note the model and serial number. They're on a label on the rating plate on the outdoor unit, and on the indoor unit or cylinder.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Daikin Altherma AA-01 error code mean?
The electric backup heater in the indoor unit got too hot and its safety cut-out has tripped, so the heat pump has stopped. The Altherma 3 R ECH2O guide words it as "Backup heater overheated or BUH power cable not connected". AA-02 is used when an external backup heater overheats. Daikin's service manual says AA-01 needs a reset at the controller and a manual reset of the backup heater's thermal protector.
Can I fix Daikin Altherma AA-01 myself?
Engineer only (engineer visit). Start with: check the water pressure; leave the cut-out to your installer; read the help text. Call your installer, who will find the cause before resetting it. Daikin's service manual lists low water pressure or flow, air in the water circuit, an external heat source, a faulty thermal protector, a faulty outlet water thermistor, a faulty backup heater or a faulty hydro PCB.
Is Daikin Altherma AA-01 worth repairing?
Worth fixing. Needs an engineer to reset the cut-out and find out why it tripped, which may be a flow or pressure problem rather than the heater itself.
Checked against
- Daikin Altherma 3 R W installer reference guide 4P496758-1B, error codes, p. 90 (ManualsLib copy)
- Daikin Altherma 3 R ECH2O installer reference guide, error codes overview, p. 274 (ManualsLib copy)
- Daikin service manual ESIE20-06A, Daikin Altherma 3 M (EBLA/EDLA), PDF (copy hosted on the OpenEnergyMonitor forum)
- Daikin Altherma 3 R ECH2O installer reference guide, AH error and help text, p. 273 (ManualsLib copy)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.