High Lockout
L1 on Keston S30

Your boiler has shut down because the water inside it has become too hot, likely due to a circulation problem.

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

Flow temperature overheat

What causes this fault?

This fault occurs when the water leaving your boiler exceeds a safe temperature limit, causing the system to shut down for safety. It is typically caused by a blockage in the pipework, a failing pump that cannot move the water quickly enough, or a build-up of debris known as sludge.

This issue often surfaces during the first cold spell of autumn, when the heating system is switched back on after being idle and internal components like the pump may have seized.

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

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