F12
Glow-worm
Gas valve control fault
F12 displayed, furnace locks out intermittently, often during high demand
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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 coil, Loose gas valve connector on Glow-worm PCB, Faulty gas valve modulation control, PCB output fault to gas valve
How to Fix / Troubleshooting
Critical safety: All gas valve testing and replacement must be carried out by a Gas Safe registered engineer. Do not attempt DIY repairs on gas components.
Technician procedure:
- Check the electrical connector at the gas valve for secure fit and signs of overheating.
- Measure coil resistance and compare with Glow-worm specifications.
- Verify that the PCB is sending the correct voltage to the gas valve during ignition and modulation.
- Check gas inlet pressure and ensure the supply is stable.
- Replace the gas valve assembly if it fails electrical or mechanical tests, and recommission the appliance with full combustion analysis.
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Repair Difficulty
Professional Required
5/5
Required Part
Gas Valve Assembly
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