High Lockout
A-A5 on Worcester Bosch Greenstar 8000 Life/Style

Your boiler has detected that the water temperature has risen too quickly or reached an unusually high level, causing it to shut down to prevent overheating.

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

Actual temperature: Flow temperature at the flow temperature sensor

What causes this fault?

This fault occurs when water cannot travel through the boiler’s heat exchanger fast enough to carry the heat away, causing a rapid temperature spike. The most common reasons are a failed internal pump, a blockage caused by sludge or debris in the pipework, or air trapped within the system preventing proper circulation.

This issue often surfaces during the first cold spell of autumn when the heating is turned on after being idle all summer, which can cause internal pump parts to seize.

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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