defrost-sensor on a Thermia heat pump: Sensor fault alternatively cable fault. Defrost sensor
Short answer
On a Thermia heat pump, defrost-sensor means: Sensor fault alternatively cable fault. Defrost sensor. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Atec
The manual says: Sensor fault alternatively cable fault. Defrost sensor.
Part involved: defrost sensor.
Likely causes:
- Sensor or cable fault. (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
On the Diplomat
The manual says: Defrost sensor fault. Heat and hot water production is instead controlled to the outdoor sensor's value (applies to Atria models).
Part involved: defrost sensor.
Is it safe?
On the Atec
| 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.
On the Diplomat
| 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, when condition clears (inferred).
What you can check
On the Atec
- Note the exact alarm text. Check the outdoor unit is not blocked by snow, ice, leaves or debris and that airflow around it is clear. Acknowledge once (operating mode OFF then back); if it remains, contact a service technician (user manual 086L0130 p.25).
On the Diplomat
- Note the exact alarm text. Check the outdoor unit is not blocked by snow, ice, leaves or debris and that airflow around it is clear. Acknowledge once by setting operating mode OFF then back; 'Other alarm message: reset the alarm; if the alarm remains, contact a service technician' (VUGFD302 p.16).
What the engineer will check
On the Atec
- Disconnect the sensor and measure resistance with and without its cable against the ohm table in 'Measurement points' (section 3.3); replace the sensor if out of tolerance, otherwise the cable.
Who to ask for: heat pump engineer.
On the Diplomat
- Measure sensor/cable resistance. Causes per the equivalent Danfoss service instructions VMGFC302 (same Danfoss-era controller) - not stated in the Thermia user manual.
Who to ask for: heat pump engineer.
What you might notice
Sources
- Service Instructions Atec (VIIFM102) - silo.tips copy ↗ · VIIFM102 · page p.10 · Reviewed 23 September 2026
- User manual Atria, Atria Duo, Atria Duo Optimum, Atria Optimum, Comfort, Diplomat, Diplomat Duo, Diplomat Duo Optimum (G2), Diplomat Optimum (G2) - 086U6297 Rev. 6 EN ↗ · 086U6297 Rev. 6 · page printed p.41 (PDF p.43) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026