421 on a NIBE heat pump: Inverter alarm type II (communication): Temporary communication alarm 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary communication alarm 3 times within 2 h or active for 1 h / Communication fault to AA2, 3 repeated communication faults within 2 hours]
Short answer
On a NIBE heat pump, 421 means: Inverter alarm type II (communication): Temporary communication alarm 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary communication alarm 3 times within 2 h or active for 1 h / Communication fault to AA2, 3 repeated communication faults within 2 hours]. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manuals for this range word it in different ways:
- Inverter alarm type II (communication): Temporary communication alarm 3 times within 2 hours or continuously for 1 hour [other NIBE documents: Temporary communication alarm 3 times within 2 h or active for 1 h / Communication fault to AA2, 3 repeated communication faults within 2 hours].
- Cold outd air, cool. mode: Outside temperature too low to ensure safe operation.
Part involved: PCB / accessory board communication; operating range / outdoor sensor.
When it shows: Temporary communication alarm 3 times within 2 hours or continuously for 1 hour; Temporary communication alarm 3 times within 2 h or active for 1 h; Communication fault to AA2, 3 repeated communication faults within 2 hours; 3 repeated communication faults within 2 hours or continuously for 1 hour Outside temperature too low to ensure safe operation.
Likely causes:
- F2120: Loose cabling | F2120: Communication with AA2-X20 interrupted; poor connection between PCB and inverter.
- S2060: Cold weather conditions; sensor fault.
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 - Compressor blocked. Manual reset in menu; Compressor blocked. Manual reset.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains - NIBE: Compressor blocked. Manual reset in menu; Compressor blocked. Manual reset.
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. Normally clears by itself when the outdoor temperature returns to the unit's operating range; check the outdoor unit is not buried in snow.
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: See outdoor unit manual; Loose cabling; Communication with AA2-X20 interrupted; poor connection between PCB and inverter.
- Check outdoor ambient sensor BT28/Tho-A value and connection; confirm installation is within operating range. NIBE lists: Cold weather conditions; sensor fault.
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 10 · Reviewed 23 September 2026
- Installer manual Air/water heat pump NIBE S2060 UK 1x230V IHB EN 2623-3 (831281) ↗ · IHB EN 2623-3 · page 32 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026