U2-00 on a Daikin Altherma: OU: Defect of power supply voltage
Short answer
On a Daikin heat pump, U2-00 means: OU: Defect of power supply voltage. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: OU: Defect of power supply voltage.
Part involved: power supply / inverter electrical.
When it shows: Power supply abnormality or instant power failure is detected.
Likely causes:
- ▪ Faulty or disturbance of the power supply (power supply MUST be within range of nominal operating voltage ±4%), ▪ Power drop, ▪ Short circuit.; Faulty compressor or miswiring of the compressor power supply cable.; Faulty outdoor unit fan motor.; Faulty main PCB.; ▪ Momentary drop of voltage, ▪ Momentary power failure.
Is it safe?
| 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: no.
Manual reset: Power reset via outdoor unit.
What you can check
- Read the full error code and sub-code shown on the user interface (Malfunctioning screen; press the help button for Daikin's long description) 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.
- Do NOT bleed radiators while an error/malfunction symbol is displayed: Daikin operation manuals warn refrigerant may have leaked into the water circuit - ventilate and call your installer.
- If heating/hot water has stopped, the operation manual lets the user confirm on the Malfunctioning screen whether the backup/booster heater may take over as emergency heating (much higher running cost) until the installer attends.
What the engineer will check
- Check if the power supply is compliant with the regulations. . ; Perform a check of the compressor. . ; Perform a check of the outdoor unit fan motor. . ; Perform a check of the main PCB. . ; Wait until the compressor restarts.
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
14 owner and engineer reports mention this.
- No heating on the Altherma: 7 reports
- Heat pump will not start on the Altherma: 5 reports
- Keeps stopping / cutting out on the Altherma: 2 reports
Sources
- Service manual Daikin Altherma 3 M EBLA/EDLA04~08E (ESIE22-09) ↗ · ESIE22-09 · page 70 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026