173 on a NIBE heat pump: Com. error heating system 2: Communication with accessory card for climate system 2 temporarily missing [other NIBE documents: Communication fault to accessory board]
Short answer
On a NIBE heat pump, 173 means: Com. error heating system 2: Communication with accessory card for climate system 2 temporarily missing [other NIBE documents: Communication fault to accessory board]. The heat pump keeps running with some functions stopped.
Reviewed 23 September 2026 · 1 source
What it means
The manuals for this range word it in different ways:
- Com. error heating system 2: Communication with accessory card for climate system 2 temporarily missing [other NIBE documents: Communication fault to accessory board].
- Pressure switch: High or low pressure switch triggered.
Part involved: PCB / accessory board communication; pressure switches/sensors, expansion valve.
When it shows: Communication with accessory card for climate system 2 temporarily missing; Communication fault to accessory board High or low pressure switch triggered.
Likely causes:
- Defective communication cables; accessory activated in display but not connected; incorrectly connected communication cable; incorrectly set DIP switch; no electrical supply to accessory board.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - Accessory 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. 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: Defective communication cables; accessory activated in display but not connected; incorrectly connected communication cable; incorrectly set DIP switch; no electrical supply to accessory board.
- Check heating medium flow, particle filter and circulation pump; check pressure switches/sensors, expansion valve, service valves and refrigerant charge (F-gas qualified).
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 15 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026