How much does it cost to fix Worcester Bosch error code C4?

Typical repair cost for the Worcester Bosch C4 fault is £120-280 including labour.

C4 High

Worcester Bosch C4 Fault Code

Q: What does the Worcester Bosch C4 fault code mean?

Your boiler has detected an issue with the air pressure inside the system, which is stopping it from safely starting the burner.

The Worcester Bosch C4 fault code means: your boiler has detected an issue with the air pressure inside the system, which is stopping it from safely starting the burner.. It appears on 24 Worcester Bosch models in our database.

Technical description: Fan running when it should be off

What causes the Worcester Bosch C4 fault?

This fault occurs when the boiler's internal safety sensor detects that the fan isn't moving enough air to safely vent exhaust gases. The most common reason is a buildup of dirt in the fan or a blockage in the small tubes that monitor air pressure, which prevents the boiler from clearing the path for ignition. In some cases, it can also be triggered by a failing fan motor or an electrical fault in the pressure sensor itself.

This fault is slightly more common during autumn and winter, as strong winds can occasionally cause pressure fluctuations in the flue, or spiders may crawl into the pressure tubes during the cooling months.

Typical repair cost £120-280 (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 27 April 2026