501 on a NIBE heat pump: Failed start, no press. diff: Pressure difference between BP9 and BP8 too low at compressor start 2 times within 30 min [other NIBE documents: Incorrect pressure set-up between BP8 and BP9 during compressor start / Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes]
Short answer
On a NIBE heat pump, 501 means: Failed start, no press. diff: Pressure difference between BP9 and BP8 too low at compressor start 2 times within 30 min [other NIBE documents: Incorrect pressure set-up between BP8 and BP9 during compressor start / Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes]. 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: Failed start, no press. diff: Pressure difference between BP9 and BP8 too low at compressor start 2 times within 30 min [other NIBE documents: Incorrect pressure set-up between BP8 and BP9 during compressor start / Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes].
Part involved: pressure switches/sensors, expansion valve.
When it shows: Pressure difference between BP9 and BP8 too low at compressor start 2 times within 30 min; Incorrect pressure set-up between BP8 and BP9 during compressor start; Pressure difference between BP9 and BP8 too low at compressor start 3 times within 30 minutes.
Likely causes:
- F2120: Problem in cooling circuit; defective pressure sensors | F2120: Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown | S2125: Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - Compressor blocked (possibly to be heated with compressor heater). Automatic 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. Air source systems: check the outdoor unit is clear of leaves/snow/ice. Read the heating system pressure gauge and note the reading for your installer.
What the engineer will check
- Check heating medium flow, particle filter and circulation pump; check pressure switches/sensors, expansion valve, service valves and refrigerant charge (F-gas qualified). NIBE lists: Fault in pressure sensor BP8 or BP9; compressor not compressing properly; compressor breakdown; Problem in cooling circuit; defective pressure sensors; Fault in pressure sensor BP8/BP9; compressor not compressing sufficiently; compressor breakdown.
Who to ask for: Heat pump engineer with F-gas qualification (NIBE-trained / NIBE PRO installer).
Certification: F-gas certification for refrigerant circuit work (plus flammable-refrigerant competence for R290/R32 models).
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 11 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026