High Lockout
C4 on Worcester Bosch 537i

Your boiler is failing to confirm that its internal fan has stopped correctly, which is preventing it from starting up safely.

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

Differential pressure switch does not open when fan is off.

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is spinning continuously even when it should be at rest. The most common reason is a mechanical failure within the fan itself or a communication error from the main circuit board that keeps the power running to the motor constantly.

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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