281 on a NIBE heat pump: Sensor fault LPT low pressure (generated by heat pump): Sensor fault, outdoor unit low pressure transmitter [other NIBE documents: Low pressure sensor LPT unreasonable value 3 times within 40 min / Repeated sensor failures, low-pressure sensor BP8 (PSL) / high-pressure sensor BP4 (PSH)]
Short answer
On a NIBE heat pump, 281 means: Sensor fault LPT low pressure (generated by heat pump): Sensor fault, outdoor unit low pressure transmitter [other NIBE documents: Low pressure sensor LPT unreasonable value 3 times within 40 min / Repeated sensor failures, low-pressure sensor BP8 (PSL) / high-pressure sensor BP4 (PSH)]. 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 manuals for this range word it in different ways:
- Sensor fault LPT low pressure (generated by heat pump): Sensor fault, outdoor unit low pressure transmitter [other NIBE documents: Low pressure sensor LPT unreasonable value 3 times within 40 min / Repeated sensor failures, low-pressure sensor BP8 (PSL) / high-pressure sensor BP4 (PSH)].
- Com. error to EB107/8-GP12: Communication to accessory card for charge pump GP12 missing.
Part involved: temperature/pressure sensor; PCB / accessory board communication.
When it shows: Sensor fault, outdoor unit low pressure transmitter; Low pressure sensor LPT unreasonable value 3 times within 40 min; Repeated sensor failures, low-pressure sensor BP8 (PSL) / high-pressure sensor BP4 (PSH); Sensor fault, low pressure transmitter in F2050; Sensor fault, low pressure transmitter in F2040 Communication to accessory card for charge pump GP12 missing.
Likely causes:
- S2060: Open/short circuit on sensor input; defective baseboard AA2; fault in refrigerant circuit | F2050: Open/short circuit on sensor input; defective control board PWB1; fault in refrigerant circuit | F2040: Open/short circuit on sensor input; defective control board; fault in refrigerant circuit.
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 - 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.
- 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 breakers for the heat pump, indoor module and any accessories are on at the consumer unit.
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; Open/short circuit on sensor input; defective baseboard AA2; fault in refrigerant circuit; Open/short circuit on sensor input; defective control board PWB1; fault in refrigerant circuit; Open/short circuit on sensor input; defective control board; fault in refrigerant circuit.
- Check communication cables and connectors between PCBs/accessory boards, DIP-switch addressing, power supply to the accessory board, and that accessories activated in the menu are actually fitted.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: Electrical work to BS 7671 by a competent person.
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 7 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026