return-line-sensor on a Thermia heat pump: Sensor fault alternatively cable fault. Return line sensor
Short answer
On a Thermia heat pump, return-line-sensor means: Sensor fault alternatively cable fault. Return line 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. Return line sensor.
Part involved: return line sensor.
Likely causes:
- Sensor or cable fault. (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
On the Diplomat
The manual says: Return sensor fault. Return temperature = Supply line - 5 is used. Calculated supply temperature limited to maximum 45°C.
Part involved: return line 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. 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. 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