BoilerFaultCodesWorcester Bosch C4 Fault Code
The Worcester Bosch C4 fault code means: your boiler has detected that the internal fan is spinning too slowly to safely clear waste gases, so it has shut down as a precaution.. It appears on 10 Worcester Bosch models in our database.
Technical description: Airflow present during last 24 hours - fan running continuously
What causes the Worcester Bosch C4 fault?
This fault occurs when the boiler's control system detects that the fan is not reaching the required speed to safely expel exhaust fumes. The most common reasons are a buildup of dirt inside the fan housing, electrical components on the fan failing, or a fault within the main circuit board that prevents it from sending the correct power to the motor.
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.
- 537i Repair: £150-250
- Greenstar 12i System Repair: £180-£450
- Greenstar 25Si Compact ErP
- Greenstar 28CDi Repair: £180-350
- Greenstar CDi Compact Repair: £150-300
- Highflow 250 Repair: £150-300
- Highflow 300 Repair: £150-300
- R30 HE Plus Repair: £150-250
- Ri 25 HE Repair: £150-350
- 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 537i manual (PDF)
- Worcester Bosch Greenstar 12i System manual (PDF)
- Worcester Bosch Greenstar 25Si Compact ErP manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026