F26
Glow-worm
Gas Valve Regulation Fault
Boiler shows F26, burner does not light, fan may run
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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 modulation coil, Incorrect gas valve calibration, Wiring fault to gas valve, PCB modulation output fault
How to Fix / Troubleshooting
Safety: Isolate gas and power before any work on gas components. Only Gas Safe engineers may adjust or replace gas valves.
- Reset and observe: If F26 appears consistently on ignition attempts, the gas valve or its control is suspect.
- Engineer tasks:
- Check gas valve wiring and connectors.
- Measure modulation coil resistance and compare to specification.
- Verify correct gas valve type and configuration for the boiler.
- Replace gas valve and/or PCB as indicated by diagnostic tests.
Do not attempt to adjust gas valve screws; incorrect settings can cause dangerous combustion.
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Repair Difficulty
Professional Required
5/5
Required Part
Modulating Gas Valve
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