High Lockout
F4 on Glow-worm Ci Range

Your boiler is lighting up but quickly switching itself off because it cannot confirm that a flame is safely present.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

FAULT: THE BURNER LIGHTS, THEN GOES OUT. The flame is spreading across the whole burner, but not being detected.

What causes this fault?

This fault occurs when the boiler's control system fails to acknowledge that a flame has successfully lit, even if you can see it burning. The most common reason is a worn or dirty flame sense electrode that can no longer send an electrical signal back to the main computer to prove it is safe to keep the gas running.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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