C1
Viessmann
Invalid Flow Temperature Data from Boiler
Thermostat displays C1 and reports that the boiler flow sensor data is invalid; control is limited.
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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
Faulty boiler flow temperature sensor, Loose sensor connection on boiler PCB, Shorted or open sensor wiring, Boiler firmware issue sending incorrect data
How to Fix / Troubleshooting
Safety first: Turn off power to the boiler before opening any access panels. Surfaces may be hot.
- Check boiler error codes: Look at the boiler display for any sensor‑related errors and follow the boiler manual.
- Inspect sensor connection: If user‑accessible, verify that the flow temperature sensor plug is firmly seated on the boiler control board.
- Monitor temperature readings: In the thermostat or boiler menu, observe the reported flow temperature. Unrealistic or fluctuating values indicate a sensor or wiring fault.
- Replace sensor: A heating engineer should replace the Viessmann flow temperature sensor if it is defective.
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Repair Difficulty
Professional Required
5/5
Required Part
Boiler flow temperature sensor
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