996 on a NIBE heat pump: Blocked (additional heat): External additional heat blocked through AUX input [other NIBE documents: Additional heat externally blocked]
Short answer
On a NIBE heat pump, 996 means: Blocked (additional heat): External additional heat blocked through AUX input [other NIBE documents: Additional heat externally blocked]. The heat pump keeps running with some functions stopped.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Blocked (additional heat): External additional heat blocked through AUX input [other NIBE documents: Additional heat externally blocked].
Part involved: AUX input / external control.
When it shows: External additional heat blocked through AUX input; Additional heat externally blocked.
Likely causes:
- External equipment connected to AUX input.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Information only |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - Additional heat blocked; automatic reset when input opens; Additional heat 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 whether any external equipment connected to the heat pump (e.g. energy supplier load control, smart thermostat or other AUX device) is signalling a block.
What the engineer will check
- Check the device connected to the AUX input and the AUX input configuration. NIBE lists: External equipment connected to AUX input.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: None specific beyond competent heat pump installer; electrical work to BS 7671.
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 21 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026