High Lockout
C4 on Worcester Bosch ZWB 7-25

Your boiler's internal fan is stuck running continuously, which prevents the system from starting a new heating cycle correctly.

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 control system detects that the fan is spinning even when it should be stationary. The most common reason is a communication error between the fan and the main circuit board, often caused by a faulty internal switch or a sticking relay that keeps the power flowing 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

C6View →

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