U2 on a Daikin heat pump: Power supply insufficient or instantaneous failure
Short answer
On a Daikin heat pump, U2 means: Power supply insufficient or instantaneous failure. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Altherma HT (high temperature split)
The manual says: Power supply insufficient or instantaneous failure.
Part involved: power supply / inverter electrical.
When it shows: When the abnormal voltage of main circuit capacitor built in the inverter and abnormal power supply voltage are detected.
Likely causes:
- Power supply insufficient; Instantaneous power failure; Defect of outdoor inverter PC board (A2P) ESIE09-08C; Defect of indoor inverter PC board [A4P (1 ø) or A5P (3ø)]; Converting problem Warning: CAPACITOR SHOCK HAZARD. The capac itors (top surfaces) can hold up to 220V. Use correct precautions when servicing. ESIE09-08C.
On the Altherma LT (low temperature split)
The manual says: Low-voltage or over-voltage detection.
Part involved: power supply / inverter electrical.
When it shows: ■ An over-voltage signal is fed from the over-voltage detection circuit to the microcomputer, or the voltage being detected by the DC voltage detection circuit is judged to be below 150 V for 0.1 second. ■ The system will be shut down if the error occurs 16 times. ■ Clearing condition: Continuous run for about 60 minutes (normal) | An over-voltage signal is fed from the over-voltage detection circuit to the microcomputer, or the voltage being detected by the DC voltage detection circuit is judged to be below 150 V for 0.1 second. The system will be shut down if the error occurs 16 times. Clearing condition: Continuous run for about 60 minutes (normal) Supply voltage not as specified Over-voltage detector or DC voltage detection circuit defective PAM control part(s) defective.
Likely causes:
- Supply voltage not as specified; Over-voltage detector or DC voltage detection circuit defective; PAM control part(s) defective.
Is it safe?
On the Altherma HT (high temperature split)
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: unknown.
On the Altherma LT (low temperature split)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: Reset via the remote controller (Inspection/Test button) after the cause is fixed; some codes need a manual reset of the remote-controller memory (service manual section 1.4).
What you can check
- Read the full error code shown on the user interface and note it, with the date/time, for your installer.
- Check the heat pump isolator switch(es) and the consumer-unit breaker/RCD for the heat pump are ON. If a breaker or RCD has tripped, do not keep resetting it - leave it and call an engineer.
What the engineer will check
On the Altherma HT (high temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check power supply insufficient; check instantaneous power failure; check defect of outdoor inverter PC board (A2P) ESIE09-08C; check defect of indoor inverter PC board [A4P (1 ø) or A5P (3ø)]; check converting problem Warning: CAPACITOR SHOCK HAZARD. The capac itors (top surfaces) can hold up to 220V. Use correct precautions when servicing. ESIE09-08C.
Who to ask for: Heat pump engineer / qualified electrician.
Certification: Electrical competence to BS 7671 (Part P where applicable) for mains supply work.
On the Altherma LT (low temperature split)
- Follow the service-manual troubleshooting flow chart for this code: check supply voltage not as specified; check over-voltage detector or DC voltage detection circuit defective; check pAM control part(s) defective.
Who to ask for: Heat pump engineer / qualified electrician.
Certification: Electrical competence to BS 7671 (Part P where applicable) for mains supply work.
What you might notice
What owners and engineers report
2 owner and engineer reports mention this.
- No heating on the Altherma LT (low temperature split): 1 report
- Heat pump will not start on the Altherma LT (low temperature split): 1 report
Sources
- Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 210 · Reviewed 23 September 2026
- Service Manual ALTHERMA ERHQ006~008BAV3 / ERLQ006~008BAV3(9) / EKHBH(X)008BA (ESIE10-02) ↗ · ESIE10-02 · page 190 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026