High Lockout
F.25 on Glow-worm Energy System

Your boiler has shut down because it detected that the exhaust gases are getting too hot, which is a safety feature to prevent internal damage.

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

Safety switch-off – flue gas temperature too high

What causes this fault?

This fault occurs when the internal safety sensors detect that the waste gases leaving your boiler have reached a dangerously high temperature. The most common reason is a buildup of debris or limescale inside the main heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be caused by a faulty sensor sending incorrect readings to the boiler's computer.

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

F.20View →

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