High Lockout
C4 on Worcester Bosch Greenstar 25 HE Combi

Your boiler's fan is not spinning fast enough to clear out exhaust fumes, so the system has shut down as a safety precaution.

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

Fan does not switch off.

What causes this fault?

This fault occurs when the boiler's internal computer detects that the fan isn't spinning at the required speed to safely extract exhaust gases from the building. The most common reasons are a build-up of dirt or debris inside the fan motor, a mechanical failure of the fan itself, or a communication issue between the fan and the main control board.

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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