F3
Viessmann
Flue Gas Temperature Sensor Fault
Boiler displays F3 and will not start, fan may run briefly
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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-circuit flue gas sensor (NTC), shorted sensor wiring, sensor out of calibration, damaged connector on main PCB
How to Fix / Troubleshooting
Safety: Overheated flue gases can be dangerous. Do not bypass or bridge any safety sensors.
- Power cycle: Turn off the boiler for 1 minute and restart to rule out a transient error.
- Inspect flue gas sensor: With power off, locate the Viessmann flue gas NTC sensor on the flue collector. Check for loose plug, damaged cable, or signs of overheating.
- Check wiring harness: Follow the sensor cable back to the control board. Ensure it is not pinched, cut, or corroded at the connector.
- Resistance check (technician): A professional can measure the NTC resistance at room temperature and compare it to Viessmann specifications. Replace the sensor if out of range.
- If wiring and sensor are OK: The fault may lie in the main control board input; board replacement should be done by an authorized service partner.
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Repair Difficulty
Hard
4/5
Required Part
Flue Gas Temperature Sensor (NTC)
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