C4 High

Worcester Bosch C4 Fault Code

The Worcester Bosch C4 fault code means: your boiler is failing to confirm that its internal fan has stopped correctly, which is preventing it from starting up safely.. It appears on 14 Worcester Bosch models in our database.

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 the Worcester Bosch C4 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.

Typical repair cost £150-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Worcester Bosch models that show the C4 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Worcester Bosch C4 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026