904 on a NIBE heat pump: High evaporation temp. (exhaust air module): Evaporation sensor BT16 in exhaust air module 50 C or higher
Short answer
On a NIBE heat pump, 904 means: High evaporation temp. (exhaust air module): Evaporation sensor BT16 in exhaust air module 50 C or higher. 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: High evaporation temp. (exhaust air module): Evaporation sensor BT16 in exhaust air module 50 C or higher.
Part involved: fans, air filters, air sensors.
When it shows: Evaporation sensor BT16 in exhaust air module 50 C or higher.
Likely causes:
- Open/short circuit on sensor input or defective sensor in exhaust air module.
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 - Accessory blocked. Manual reset.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains - NIBE: Accessory blocked. Manual reset.
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 exhaust air valves/grilles in the rooms are open and not blocked; check the filter reminder in the user manual maintenance section.
What the engineer will check
- Check ventilation airflow, fans, filters, air sensors and condensate drainage/level monitor. NIBE lists: Open/short circuit on sensor input or defective sensor in exhaust air module.
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 12 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026