F6
Viessmann
Flow temperature sensor fault
Furnace displays supply temperature sensor error, unit may not fire or may run erratically
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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
Open or shorted flow temperature sensor, Loose sensor clip on flow pipe, Damaged sensor cable, Faulty control board analog input
How to Fix / Troubleshooting
Safety: Isolate power and allow the system to cool before handling sensors.
- Locate the sensor: On Viessmann furnaces, the flow temperature sensor (NTC) is usually clipped to the flow pipe leaving the heat exchanger. Confirm it is firmly attached and making good contact with the pipe.
- Inspect wiring: Check the cable for cuts, kinks, or signs of overheating. Reseat the connector on the main control board.
- Check for water leaks: Look for any leaks around the sensor pocket or pipe that could damage the sensor or wiring.
- Reset and observe: Restore power and reset the unit. If F6 reappears immediately, the sensor is likely defective and should be replaced with a genuine Viessmann NTC.
- Technician verification: A professional can measure the resistance of the NTC and compare it to the Viessmann temperature/resistance chart to confirm failure.
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Repair Difficulty
Medium
3/5
Required Part
Flow Temperature Sensor (NTC)
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