F4
FAULT CODE
Glow-worm
Gas Supply / Ignition Lockout
Medium Severity
DIY Check Possible
The boiler has failed to ignite after five attempts — typically indicating a gas supply issue or condensate pipe blockage.
Cold Weather Note
Check for frozen condensate pipe in cold weather first.
Step-by-Step Fix Guide
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Step 1
Check the gas supply — try a gas hob or other appliance
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Step 2
Check for a frozen condensate pipe
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Step 3
Reset and try again once
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Step 4
If the fault persists, call a Gas Safe engineer to check the gas valve and ignition system
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.