420 on a NIBE heat pump: Inverter alarm type II (temporary communication): A temporary communication alarm with the inverter has occurred [other NIBE documents: Temporary communication fault in the inverter / Communication fault to AA2]
Short answer
On a NIBE heat pump, 420 means: Inverter alarm type II (temporary communication): A temporary communication alarm with the inverter has occurred [other NIBE documents: Temporary communication fault in the inverter / Communication fault to AA2]. The heat pump keeps running with some functions stopped.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Inverter alarm type II (temporary communication): A temporary communication alarm with the inverter has occurred [other NIBE documents: Temporary communication fault in the inverter / Communication fault to AA2].
Part involved: PCB / accessory board communication.
When it shows: A temporary communication alarm with the inverter has occurred; Temporary communication fault in the inverter; Communication fault to AA2.
Likely causes:
- F2120: Loose cabling.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - Compressor stopped; automatic reset 60 s after inverter fault reset; Compressor stopped; restart attempt 60 s after inverter fault reset.
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: Main and group fuses and inverter cable/connections; communication cable between PCB and inverter; Loose cabling.
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 18 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026