3 on a NIBE heat pump: Sensor fault BT3: Sensor not connected/defective (heating medium return) [other NIBE documents: Sensor fault, incoming water sensor in F2050 (BT3) / Sensor fault, incoming water sensor in F2040 (BT3)]
Short answer
On a NIBE heat pump, 3 means: Sensor fault BT3: Sensor not connected/defective (heating medium return) [other NIBE documents: Sensor fault, incoming water sensor in F2050 (BT3) / Sensor fault, incoming water sensor in F2040 (BT3)]. 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: Sensor fault BT3: Sensor not connected/defective (heating medium return) [other NIBE documents: Sensor fault, incoming water sensor in F2050 (BT3) / Sensor fault, incoming water sensor in F2040 (BT3)].
Part involved: temperature/pressure sensor.
When it shows: Sensor not connected/defective (heating medium return); Sensor fault, incoming water sensor in F2050 (BT3); Sensor fault, incoming water sensor in F2040 (BT3).
Likely causes:
- F2050: Open/short circuit on sensor input; sensor not working; defective control board PWB1 | F2040: Open/short circuit on sensor input; sensor not working; defective control board AA23.
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: no/unknown - Compressor blocked during hot water charging. VVM 500: heating medium pump runs at speed set in menu 5.1.19.
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.
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: Open/short circuit on sensor input; sensor not working; defective control board PWB1; Open/short circuit on sensor input; sensor not working; defective control board AA23.
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
- ALARM LIST - NIBE products with Emmy display, Version 1747-1 (updated 2017-11-23) ↗ · 1747-1 · page 1 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026