400 on a NIBE heat pump: Unspecified fault: Unspecified faults from outdoor unit [other NIBE documents: Initiation fault inverter / inverter not compatible / configuration file missing / charge error configuration]
Short answer
On a NIBE heat pump, 400 means: Unspecified fault: Unspecified faults from outdoor unit [other NIBE documents: Initiation fault inverter / inverter not compatible / configuration file missing / charge error configuration]. 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 manual says: Unspecified fault: Unspecified faults from outdoor unit [other NIBE documents: Initiation fault inverter / inverter not compatible / configuration file missing / charge error configuration].
Part involved: PCB / software configuration.
When it shows: Unspecified faults from outdoor unit; Initiation fault inverter / inverter not compatible / configuration file missing / charge error configuration.
Likely causes:
- F2120: Inverter not compatible; configuration fault | S2125: Inverter not compatible; configuration fault.
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 - Master blocks relevant compressor; frost protection safeguarded.
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. Note the alarm and contact your installer; no homeowner action beyond resetting.
What the engineer will check
- Check serial number/software versions, PCB type and DIP switch settings; update software or replace PCB as required. NIBE lists: Outdoor unit software could be more recent than indoor unit's alarm list; Inverter not compatible; configuration fault.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: 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 9 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026