High Lockout
BC on Worcester Bosch Greenstar Utility 18/25

Your boiler has shut down because it is not burning fuel efficiently or safely, which is usually caused by a faulty internal sensor or an incorrect gas mixture.

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

Combustion settings incorrect

What causes this fault?

This fault occurs when the boiler's internal monitoring system detects that the flame is not burning at the correct intensity or stability required for safe operation. The most common reason is that the sensing electrodes have become worn or dirty, preventing the boiler from 'seeing' the flame even if it is lit. It can also be triggered if the gas valve is not delivering the precise amount of fuel needed for clean combustion.

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

C1View →C6View →

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