High Lockout
Illuminated reset light on Glow-worm Micron 50FF

Your boiler has switched itself off because it has reached an unsafe temperature and needs to be cooled down and reset.

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 Temperature Limiter (overheat) operates. Turn temperature control to '0' and then back to original position.

What causes this fault?

This fault occurs when the water inside the heat exchanger becomes too hot, triggering a safety switch to prevent damage. The most common reasons are a pump failure that stops water from moving around the system or a build-up of sludge that creates a blockage.

This issue often appears when the heating is first turned on after summer, as the pump can seize up after long periods of 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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