353 on a NIBE heat pump: Uncertain sensor accuracy (BT3/BT12): Heating medium sensors BT3 and BT12 differ by more than 2 K at calibration
Short answer
On a NIBE heat pump, 353 means: Uncertain sensor accuracy (BT3/BT12): Heating medium sensors BT3 and BT12 differ by more than 2 K at calibration. 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:
- Uncertain sensor accuracy (BT3/BT12): Heating medium sensors BT3 and BT12 differ by more than 2 K at calibration.
- Failed start, no press. diff: Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes.
Part involved: temperature/pressure sensor; pressure switches/sensors, expansion valve.
When it shows: Heating medium sensors BT3 and BT12 differ by more than 2 K at calibration Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes.
Likely causes:
- S2125: Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown | F2120: Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown.
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 - GP1 switches to manual speed; manual reset in menu 5.1.11.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains - NIBE: GP1 switches to manual speed; manual reset in menu 5.1.11.
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. 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 sensor and its connections/wiring against the wiring diagram; measure sensor resistance against the NIBE temperature/resistance table; check the relevant PCB input.
- 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: Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP); Heat pump engineer with F-gas qualification (NIBE-trained / NIBE PRO installer).
Certification: Electrical work to BS 7671 by a competent person; F-gas certification for refrigerant circuit work (plus flammable-refrigerant competence for R290/R32 models).
What you might notice
Sources
- ALARM LIST - NIBE products with Emmy display, Version 1747-1 (updated 2017-11-23) ↗ · 1747-1 · 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