C6
Worcester Bosch Boiler
Severity: ModerateWhat Does This Error Mean?
Worcester Bosch C6 means the fan is running too fast or too slow compared to what the PCB commanded.
The fan must rotate within a tight band so the burner gets the right air-fuel mix.
Cause is most often a flue obstruction, a worn fan motor, or a loose harness at the fan connector.
Less commonly, the PCB driver itself has failed.
Affected Models
- Worcester Bosch Greenstar 30i
- Worcester Bosch Greenstar 25i
- Worcester Bosch Greenstar 8000 Life
- Worcester Bosch Greenstar 8000 Style
- Worcester Bosch CDi Compact
- Worcester Bosch Highflow
Common Causes
- Flue blocked at termination
- Bird nest or leaves at horizontal flue tip
- Fan blade clogged with combustion residue
- Hall sensor inside fan failed
- Fan motor bearings worn from age
- Connector at the fan loose after vibration
How to Fix It
-
Check flue termination.
Walk to the flue outside.
Look for nests, leaves, or icing in cold weather.
Clear gently.
This single step resolves a notable fraction of C6 reports. -
Reset the boiler.
Hold the reset button on the front of the boiler for a few seconds.
The boiler attempts a fresh ignition.
If C6 returns, the fault is internal. -
Engineer step: open the boiler.
A Gas Safe engineer powers off, removes the front cover, and inspects the fan.
Visible dust on the blades or condensate residue around the housing reduce fan speed. -
Engineer step: reseat fan harness.
Pin corrosion at the fan connector is common in older boilers.
Disconnect, clean with electrical contact cleaner, and reseat.
Many C6 cases clear here without parts. -
Engineer step: test fan rotation.
With power off, the fan should rotate freely with light resistance.
Stiff or grinding = bearing failure.
Worcester replacement fan assemblies are 150-300 GBP plus engineer labour. -
Engineer step: PCB check.
If fan, harness, and flue all check out, the PCB fan-driver section has failed.
Boards are 200-400 GBP and must match the boiler serial range. -
Final test.
After repair, run a heat call.
The fan should ramp to commanded speed within seconds.
No C6 across multiple cycles confirms the repair held.
Frequently Asked Questions
Will the boiler still produce hot water with C6?
No — Worcester locks out when C6 is active because combustion airflow is wrong.
You will have no heating and no hot water until repair.
Plan accordingly during cold weather.