Heat pumps · Error codes · Thermia
failsafe-modeThermia · Atec
failsafe-mode on a Thermia Atec: Sensor fault alternatively cable fault on one of the refrigerant sensors. The heat pump goes to failsafe mode and defrosts on time
Short answer
On a Thermia heat pump, failsafe-mode means: Sensor fault alternatively cable fault on one of the refrigerant sensors. The heat pump goes to failsafe mode and defrosts on time. 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 alternatively cable fault on one of the refrigerant sensors. The heat pump goes to failsafe mode and defrosts on time.
Part involved: refrigerant sensors.
Likely causes:
- Refrigerant sensor or cable fault. (causes from the equivalent Danfoss DHP-AQ service instructions VMGFJ202).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
What you can check
- 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).
What the engineer will check
- 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.
What you might notice
Sources
- Service Instructions Atec (VIIFM102) - silo.tips copy ↗ · VIIFM102 · page p.10 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026