321 on a NIBE heat pump: Comm. fault with EB107/8-GP12: Communication with accessory board for GP12 missing for 15 s
Short answer
On a NIBE heat pump, 321 means: Comm. fault with EB107/8-GP12: Communication with accessory board for GP12 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 · 1 source
What it means
The manual says: Comm. fault with EB107/8-GP12: Communication with accessory board for GP12 missing for 15 s.
Part involved: PCB / accessory board communication.
When it shows: Communication with accessory board for GP12 missing for 15 s.
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 | 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 - Relevant compressor blocked and GP12 stopped.
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.
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.
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 9 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026