Daikin Altherma fault AJ-03

What it means

Triggered when the domestic hot water heat-up time exceeds 6 hours. Unit will switch to space heating/cooling for 3 hours.

Likely causes

1. Faulty backup heater
2. Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%)
3. Power drop
4. Short circuit
5. Faulty hydro PCB
6. Faulty 3-way valve
7. Leaking field installed domestic hot water tap
8. Backup heater NOT allowed
9. Mismatch between the software ID and EEPROM on the PCB or user interface
10. Domestic hot water consumption too large
11. Deviation of thermistor (higher temperature measured than actual).
12. Bad contact between sensor and pipe.
13. Deviation of outlet water after backup heater thermistor (lower temperature measured than actual)
14. Deviation of thermistor (lower temperature measured than actual)

Where to start

1. Perform a check of the backup heater
2. Check if the power supply is compliant with the regulations
3. Perform a check of the hydro PCB