F18
Viessmann
Internal power supply / low voltage fault
Power supply or internal voltage error, boiler may reboot or lock out intermittently
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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
Unstable mains voltage, Loose power connections, Failing internal power supply on control board, Water ingress on electronics
How to Fix / Troubleshooting
Safety: Isolate the boiler from the mains before opening the electrical compartment.
- Check mains supply: Verify that the boiler is on a dedicated circuit and that there are no obvious issues with the breaker or wiring.
- Inspect terminal block: Open the electrical cover and check that live, neutral, and earth connections are tight and free of discoloration.
- Look for moisture: Inspect the control board area for signs of condensation or leaks. If water is present, do not power the unit; locate and fix the leak and allow components to dry.
- Surge protection: Consider installing surge protection if the local supply is unstable.
- Board replacement: If F18 persists and mains supply is stable, the control board power supply may be failing and should be replaced by a technician.
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Repair Difficulty
Hard
4/5
Required Part
Main Control Board
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