F6
Viessmann
Flow Temperature Sensor Fault
Boiler shows F6, heating and hot water disabled
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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 NTC sensor, damaged sensor wiring, poor contact at PCB connector, sensor out of calibration
How to Fix / Troubleshooting
Safety: Allow the boiler to cool before touching any pipework or sensors.
- Check system pressure: Ensure the heating circuit is filled and bled; very low water can cause abnormal readings.
- Inspect flow sensor: Locate the Viessmann flow NTC on the flow pipe leaving the heat exchanger. Check for loose plug or corrosion.
- Wiring check: Follow the cable back to the control board, ensuring no damage or signs of overheating.
- Sensor replacement: The flow NTC is usually a simple screw-in or clip-on part. With the system drained or isolated, it can be replaced by a competent person following the manual.
- If F6 persists after replacement: The main PCB input may be defective and should be tested and replaced by an authorized technician.
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Repair Difficulty
Medium
3/5
Required Part
Flow Temperature Sensor (NTC)
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