494 on a NIBE heat pump: Preheat. failed, long run time: The inverter has tried to heat up the compressor but has failed
Short answer
On a NIBE heat pump, 494 means: Preheat. failed, long run time: The inverter has tried to heat up the compressor but has failed. 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: Preheat. failed, long run time: The inverter has tried to heat up the compressor but has failed.
Part involved: compressor discharge (hot gas).
When it shows: The inverter has tried to heat up the compressor but has failed.
Likely causes:
- S2125: Defective inverter; discharge sensor BT14 has come loose from its mount.
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 - not stated.
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 discharge sensor BT14 mounting, refrigerant charge and circuit (F-gas qualified), airflow over evaporator and heating medium flow. NIBE lists: Defective inverter; discharge sensor BT14 has come loose from its mount.
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
- Installer manual Air/water heat pump NIBE S2125 UK 1x230V IHB EN 2525-1 (831874) ↗ · IHB EN 2525-1 · page 41 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026