C2
Viessmann
Return or Buffer Temperature Sensor Fault (Reported)
Thermostat shows C2 and cannot read return temperature or buffer tank temperature from the boiler/heat pump.
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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
Disconnected return/buffer sensor, Faulty Viessmann NTC sensor, Wiring damage between sensor and controller, Incorrect sensor assignment in boiler configuration
How to Fix / Troubleshooting
Safety first: Power off the boiler/heat pump before accessing internal wiring. Pipes and sensors may be hot.
- Check boiler display: Confirm which sensor is reported faulty (return, buffer, or other) using the boiler’s diagnostic menu.
- Inspect sensor wiring: Ensure the sensor leads are firmly connected to the correct terminals and that the cable is not damaged.
- Verify configuration: In installer settings on the boiler, confirm that the sensor is enabled and assigned to the correct function.
- Replace sensor if needed: Have a technician replace the affected Viessmann sensor if readings are missing or unrealistic.
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Repair Difficulty
Professional Required
5/5
Required Part
Viessmann NTC temperature sensor (return/buffer)
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