231 on a NIBE heat pump: Phase sequence error: Heat pump (outdoor unit) sends phase fault message to the controller [other NIBE documents: Alarm 18 from outdoor unit]
Short answer
On a NIBE heat pump, 231 means: Phase sequence error: Heat pump (outdoor unit) sends phase fault message to the controller [other NIBE documents: Alarm 18 from outdoor unit]. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manuals for this range word it in different ways:
- Phase sequence error: Heat pump (outdoor unit) sends phase fault message to the controller [other NIBE documents: Alarm 18 from outdoor unit].
- Motor protection alarm: Heat pump (selected outdoor unit) sends fault message to controller.
Part involved: inverter and compressor drive.
When it shows: Heat pump (outdoor unit) sends phase fault message to the controller; Alarm 18 from outdoor unit Heat pump (selected outdoor unit) sends fault message to controller.
Likely causes:
- See relevant outdoor unit manual.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no/unknown - Compressor blocked.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains.
What you can check
- Read the alarm text and the 'info/action' help on the controller display (VVM/SMO or S-series smartguide). NIBE's manuals let the homeowner select 'reset alarm' ('Reset alarm and try again' on S-series); if the lamp returns to normal the alarm has cleared. If it comes back, do not keep resetting: select 'aid mode' ('Auxiliary operation' on S-series) if offered, so any electric additional heat keeps heat/hot water going, and contact your installer. Check the heat pump and indoor unit breakers/RCD at the consumer unit are on; if one trips again, leave it off and call an engineer.
What the engineer will check
- Check main and group fuses and cable connections, supply voltage under load, inverter terminal blocks, inverter mounting/cooling paste, compressor wiring and compressor. NIBE lists: See relevant outdoor unit manual.
- Check main and group fuses and cable connections, supply voltage under load, inverter terminal blocks, inverter mounting/cooling paste, compressor wiring and compressor.
Who to ask for: NIBE-trained heat pump engineer and/or qualified electrician.
Certification: Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales).
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 6 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026