High Lockout
F5 on Glow-worm Xtrafast 120

Your boiler has switched itself off because it has become too hot inside, likely due to a circulation issue or a temporary blockage.

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

Overheat thermostat tripped

What causes this fault?

This fault occurs when the internal temperature of the boiler exceeds safe limits, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving or a buildup of debris and limescale that restricts flow through the heat exchanger.

This issue often surfaces in autumn when the heating is turned back on after a long summer break, as internal pump components can become seized from inactivity.

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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