C7
Viessmann
Heat Pump Defrost Sensor Fault (Reported)
Thermostat displays C7 and indicates that the heat pump defrost sensor data is invalid (for heat pump systems).
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Defective evaporator/defrost temperature sensor, Loose sensor connection at heat pump controller, Damaged sensor wiring exposed outdoors, Ice or contamination affecting sensor contact
How to Fix / Troubleshooting
Safety first: Do not open the sealed sections of the heat pump. Work only on user‑accessible panels and menus. Outdoor units may have sharp fins and moving fans.
- Check heat pump display: Review the error details on the heat pump controller to confirm which sensor is affected.
- Inspect visible wiring: From outside, look for obvious damage to sensor cables near the evaporator coil area.
- Allow defrost cycle: If heavy ice buildup is present, allow the unit to complete a defrost cycle or gently remove loose snow/ice from around the unit (do not use sharp tools).
- Call a heat pump technician: Sensor replacement and internal checks must be done by a qualified technician familiar with Viessmann heat pumps.
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Repair Difficulty
Professional Required
5/5
Required Part
Heat pump defrost temperature sensor
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