F12
Glow-worm
Communication Fault Between User Interface and Main PCB
Boiler shows F12, may not respond to controls, display may be frozen
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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
Loose ribbon cable between display and PCB, Faulty user interface board, Faulty main PCB, Moisture ingress on control electronics
How to Fix / Troubleshooting
Safety: Isolate mains power before accessing the control panel.
- Power cycle: Turn off the boiler at the fused spur for 2–3 minutes, then restore power. This can clear transient communication errors.
- Check for condensation: Inspect around the control panel for signs of moisture or leaks from above (e.g., from pipework). Address any leaks immediately.
- Engineer tasks:
- Remove the front panel and check the ribbon cable or harness between the display and main PCB is fully seated.
- Inspect both boards for burn marks or corrosion.
- Test and replace the user interface PCB or main PCB as required.
Control board replacement should be carried out by a competent engineer to ensure correct configuration.
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Repair Difficulty
Hard
4/5
Required Part
User Interface PCB
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