425 on a NIBE heat pump: Triggered pressure switch: High or low pressure switch (or compressor over-temperature FQ) triggered [other NIBE documents: Permanent pressure switch alarm / 2 repeated LP/HP pressure switch alarms within 2.5 hours]
Short answer
On a NIBE heat pump, 425 means: Triggered pressure switch: High or low pressure switch (or compressor over-temperature FQ) triggered [other NIBE documents: Permanent pressure switch alarm / 2 repeated LP/HP pressure switch alarms within 2.5 hours]. 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: Triggered pressure switch: High or low pressure switch (or compressor over-temperature FQ) triggered [other NIBE documents: Permanent pressure switch alarm / 2 repeated LP/HP pressure switch alarms within 2.5 hours].
Part involved: pressure switches/sensors, expansion valve.
When it shows: High or low pressure switch (or compressor over-temperature FQ) triggered; Permanent pressure switch alarm; 2 repeated LP/HP pressure switch alarms within 2.5 hours; 2 repeated LP/HP/FQ alarms within 2.5 hours.
Likely causes:
- F2120: Poor heating medium flow; refrigerant leak | F2120: Poor heating medium flow; lack of refrigerant; FQ14 compressor peak high temperature 120 C | S2125: Poor heating medium flow; lack of refrigerant; FQ14 high compressor peak temperature (S2125-8/-12: 120 C; S2125-16/-20 FQ14.1: 130 C).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| 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. Air source systems: check the outdoor unit is clear of leaves/snow/ice. Read the heating system pressure gauge and note the reading for your installer.
What the engineer will check
- Check heating medium flow, particle filter and circulation pump; check pressure switches/sensors, expansion valve, service valves and refrigerant charge (F-gas qualified). NIBE lists: Check heating medium flow; lack of refrigerant; Poor heating medium flow; refrigerant leak; Poor heating medium flow; lack of refrigerant; FQ14 compressor peak high temperature 120 C; Poor heating medium flow; lack of refrigerant; FQ14 high compressor peak temperature (S2125-8/-12: 120 C; S2125-16/-20 FQ14.1: 130 C).
Who to ask for: Heat pump engineer with F-gas qualification (NIBE-trained / NIBE PRO installer).
Certification: F-gas certification for refrigerant circuit work (plus flammable-refrigerant competence for R290/R32 models).
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
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026