High Lockout
C4 on Worcester Bosch Greenstar 12i System

Your boiler has detected that the internal fan is spinning too slowly to safely clear waste gases, so it has shut down as a 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

Airflow present during last 24 hours - fan running continuously

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is not reaching the required speed to safely expel exhaust fumes. The most common reasons are a buildup of dirt inside the fan housing, electrical components on the fan failing, or a fault within the main circuit board that prevents it from sending the correct power to the motor.

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 →A1View →

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