303 on a NIBE heat pump: Com. error slave 3: Temporarily no communication with slave heat pump (EB103) [other NIBE documents: Communication with slave heat pump 2-8 missing for 15 s]
Short answer
On a NIBE heat pump, 303 means: Com. error slave 3: Temporarily no communication with slave heat pump (EB103) [other NIBE documents: Communication with slave heat pump 2-8 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 manuals for this range word it in different ways:
- Com. error slave 3: Temporarily no communication with slave heat pump (EB103) [other NIBE documents: Communication with slave heat pump 2-8 missing for 15 s].
- Internal OPT error: Gas boiler (GBM) alarm; try resetting, if it recurs see GBM manual.
Part involved: PCB / accessory board communication.
When it shows: Temporarily no communication with slave heat pump (EB103); Communication with slave heat pump 2-8 missing for 15 s Gas boiler (GBM) alarm; try resetting, if it recurs see GBM manual.
Likely causes:
- Check communication cables.
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 - Slave compressor blocked; Compressor in relevant heat pump 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.
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 communication cables.
- See NIBE alarm list and relevant installer/service manual.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: Electrical work to BS 7671 by a competent person; None specific beyond competent heat pump installer; electrical work to BS 7671.
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 8 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026