BoilerFaultCodesWorcester Bosch C4 Fault Code
The Worcester Bosch C4 fault code means: your boiler's fan is running constantly or at the wrong time, which is usually caused by a safety component failing to tell the fan when to stop.. It appears on 10 Worcester Bosch models in our database.
Technical description: Fan does not switch off.
What causes the Worcester Bosch C4 fault?
This fault occurs when the boiler's control system detects that the fan is spinning even when it shouldn't be, or the air pressure switch is stuck in the 'on' position. The most common reason is a mechanical failure within the air pressure switch or a communication error from the main circuit board that prevents the fan from shutting down safely.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A cover plan would handle faults like C4: no call-out fees, no surprise bills.
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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.
- 24/28/35 CDi MkII Repair: £150-250
- 537i Repair: £150-250
- Greenstar 12i System Repair: £180-£450
- Greenstar 2000 Repair: £180-350
- Greenstar 27CDi Repair: £180-350
- Greenstar 28CDi Repair: £180-350
- Greenstar 30HE Repair: £180-350
- Greenstar CDi Compact Repair: £150-300
- R30 HE Plus Repair: £150-250
- Worcester 25/30 HE Repair: £180-450
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.
- Worcester Bosch 24/28/35 CDi MkII manual (PDF)
- Worcester Bosch 537i manual (PDF)
- Worcester Bosch Greenstar 12i System manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026