337 on a NIBE heat pump: Sen flt: EP45 BT2 supply temp sens.: Sensor not connected/defective
Short answer
On a NIBE heat pump, 337 means: Sen flt: EP45 BT2 supply temp sens.: Sensor not connected/defective. The manual does not say whether the heat pump stops. 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:
- Sen flt: EP45 BT2 supply temp sens.: Sensor not connected/defective.
- High power load, compr.: Power output from inverter too high 3 times within 2 h or continuously 1 h.
Part involved: temperature/pressure sensor; inverter and compressor drive.
When it shows: Sensor not connected/defective Power output from inverter too high 3 times within 2 h or continuously 1 h.
Likely causes:
- Open circuit or short circuit on sensor input; defective sensor.
- S2125: Disruption in supply voltage; poor heating medium flow; defective compressor | F2120: Disruption in supply voltage; poor heating medium flow; defective compressor.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: no/unknown - Signal replaced by EP45-BT3 - 10K during shunt control.
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.
- 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.
What the engineer will check
- Check sensor and its connections/wiring against the wiring diagram; measure sensor resistance against the NIBE temperature/resistance table; check the relevant PCB input. NIBE lists: Open circuit or short circuit on sensor input; defective sensor.
- 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: Disruption in supply voltage; poor heating medium flow; defective compressor.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP); NIBE-trained heat pump engineer and/or qualified electrician.
Certification: Electrical work to BS 7671 by a competent person; Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales).
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
- 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