437 on a NIBE heat pump: Inverter alarm type II / mains disturbance: Temporary inverter fault 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary inverter fault 3 times within 2 h or continuous 1 h / High current to inverter 3 times within 2 hours]
Short answer
On a NIBE heat pump, 437 means: Inverter alarm type II / mains disturbance: Temporary inverter fault 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary inverter fault 3 times within 2 h or continuous 1 h / High current to inverter 3 times within 2 hours]. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manuals for this range word it in different ways:
- Inverter alarm type II / mains disturbance: Temporary inverter fault 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary inverter fault 3 times within 2 h or continuous 1 h / High current to inverter 3 times within 2 hours].
- Incorrect PCB in heat pump: The heat pump has the wrong baseboard.
Part involved: inverter and compressor drive; PCB / software configuration.
When it shows: Temporary inverter fault 3 times within 2 hours or continuously for 1 hour; Temporary inverter fault 3 times within 2 h or continuous 1 h; High current to inverter 3 times within 2 hours; Temporary fault in inverter, 3 times within 2 hours or continuously for 1 hour; Temporary fault in inverter, 3 times within 2 hours or continuously 1 hour The heat pump has the wrong baseboard.
Likely causes:
- F2120: Overcurrent protection tripped (1x230V products) | F2120: Disruption in supply voltage; incorrect connection in inverter terminal block X1 | S2125: Disruption in supply voltage; incorrect connection in inverter terminal block X1.
- S2125: Baseboard replaced with one not intended for this product.
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 in menu; Compressor blocked.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains - NIBE: Compressor blocked. Manual reset in menu; Compressor blocked.
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.
- 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. Note the alarm and contact your installer; no homeowner action beyond resetting.
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: Main and group fuses and cable connections; incorrect connection in inverter terminal block X5; Overcurrent protection tripped (1x230V products); Disruption in supply voltage; incorrect connection in inverter terminal block X1.
- Check serial number/software versions, PCB type and DIP switch settings; update software or replace PCB as required. NIBE lists: Baseboard replaced with one not intended for this product.
Who to ask for: NIBE-trained heat pump engineer and/or qualified electrician; NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales); None specific beyond competent heat pump installer; electrical work to BS 7671.
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 10 · Reviewed 23 September 2026
- Installer manual Air/water heat pump NIBE S2125 UK 1x230V IHB EN 2525-1 (831874) ↗ · IHB EN 2525-1 · page 42 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026