High Lockout
F25 on Glow-worm Flexicom sx

Your boiler has switched itself off because the waste gases are getting too hot, which is usually caused by a lack of water circulating through the system.

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

Flue gas temperature too high

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are getting dangerously hot, triggering a safety shutdown to prevent damage. The most common reason is that heat isn't being carried away from the burner quickly enough, usually because the water isn't circulating properly or there is a blockage within the boiler's pipework.

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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Related fault codes

F22View →F24View →F20View →

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