F9
Viessmann
Gas Valve / Modulation Fault
Boiler locks out with F9, no heating or hot water
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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
Defective gas valve actuator, wiring fault between gas valve and PCB, incorrect gas valve calibration, control board output failure
How to Fix / Troubleshooting
Safety: Gas valve faults must only be handled by qualified gas technicians.
- Do not attempt DIY gas work: Do not remove or adjust the gas valve yourself.
- Check supply pressure (technician): A professional should measure inlet and burner pressure and compare with Viessmann data plate values.
- Inspect wiring: Ensure the gas valve plug is fully seated and free of corrosion.
- Valve replacement: If the valve does not respond to modulation commands, it must be replaced and set up using a flue gas analyser according to Viessmann commissioning instructions.
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Repair Difficulty
Professional Required
5/5
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