251 on a NIBE heat pump: Com. error ACC Modbus 40: Temporarily no communication with MODBUS 40 accessory card [other NIBE documents: Communication with MODBUS accessory missing for 15 s]
Short answer
On a NIBE heat pump, 251 means: Com. error ACC Modbus 40: Temporarily no communication with MODBUS 40 accessory card [other NIBE documents: Communication with MODBUS accessory missing for 15 s]. The heat pump keeps running with some functions stopped. 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:
- Com. error ACC Modbus 40: Temporarily no communication with MODBUS 40 accessory card [other NIBE documents: Communication with MODBUS accessory missing for 15 s].
- High HWX temp: High pressure and/or temperature in the evaporator [other NIBE documents: Temperature deviation on heat exchanger sensor (Tho-R1/R2) five times within 60 min or 60 min continuously / Temperature deviation on heat exchanger sensor (Tho-R1/R2) five times within 60 min or continuously 60 min].
Part involved: PCB / accessory board communication; pressure switches/sensors, expansion valve.
When it shows: Temporarily no communication with MODBUS 40 accessory card; Communication with MODBUS accessory missing for 15 s High pressure and/or temperature in the evaporator; Temperature deviation on heat exchanger sensor (Tho-R1/R2) five times within 60 min or 60 min continuously; Temperature deviation on heat exchanger sensor (Tho-R1/R2) five times within 60 min or continuously 60 min.
Likely causes:
- Check the communication.
- S2060: Sensor not working; insufficient air circulation or blocked heat exchanger; defective control board; too much refrigerant | F2050: Sensor not working; insufficient air circulation or blocked heat exchanger; defective PWB1; too much refrigerant.
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; 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; 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 breakers for the heat pump, indoor module and any accessories are on at the consumer unit.
- 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 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. NIBE lists: Check the communication.
- 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: Sensor not working; insufficient air circulation or blocked heat exchanger; defective control board; too much refrigerant; Sensor not working; insufficient air circulation or blocked heat exchanger; defective PWB1; too much refrigerant.
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
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 6 · Reviewed 23 September 2026
- Installer manual Air/water heat pump NIBE S2060 UK 1x230V IHB EN 2623-3 (831281) ↗ · IHB EN 2623-3 · page 31 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026