366 on a NIBE heat pump: Sensor fault EPxx, BT3 return line sensor (listed as range 361-368): BT3 (return) not connected/defective in one of the climate systems
Short answer
On a NIBE heat pump, 366 means: Sensor fault EPxx, BT3 return line sensor (listed as range 361-368): BT3 (return) not connected/defective in one of the climate systems. The heat pump keeps running.
Reviewed 23 September 2026 · 1 source
What it means
The manuals for this range word it in different ways:
- Sensor fault EPxx, BT3 return line sensor (listed as range 361-368): BT3 (return) not connected/defective in one of the climate systems.
- Display: The display has restarted.
Part involved: temperature/pressure sensor; display / control unit.
When it shows: BT3 (return) not connected/defective in one of the climate systems The display has restarted.
Likely causes:
- Sensor not connected; open-circuit or defective sensor.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
The heat pump keeps running.
Resetting itself: yes - -.
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.
- 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. No action needed unless it happens repeatedly; check for power cuts or tripping breakers.
What the engineer will check
- Check sensor and its connections/wiring against the wiring diagram; measure sensor resistance against the NIBE temperature/resistance table; check the relevant PCB input. NIBE lists: Sensor not connected; open-circuit or defective sensor.
- Check for supply interruptions or intermittent connections if restarts are frequent.
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 18 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026