The fan is not running at the speed the boiler expects, so it will not light for safety.
Most common on: Greenstar i, CDi Classic
C6 means the control board is not getting the correct speed signal back from the fan. Because the fan drives combustion air and clears flue gases, the boiler will not start ignition until it confirms the fan is running correctly.
It is often a failing fan, a wiring/connector fault, or a blockage affecting the fan or flue.
Occasionally a one-off glitch clears with a reset. If C6 returns, the fan or its wiring needs checking.
Visually check the external flue terminal is not blocked by debris or nesting. Do not dismantle the flue yourself.
The fan speed signal, wiring and PCB need testing by a Gas Safe engineer — this is not a DIY fix.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The fan is not running at the speed the boiler expects, so it will not light for safety. C6 means the control board is not getting the correct speed signal back from the fan. Because the fan drives combustion air and clears flue gases, the boiler will not start ignition until it confirms the fan is running correctly.
You can try a single reset and the safe checks below, but Worcester Bosch C6 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring C6 means the underlying cause has not been fixed — most often worn or seized fan. Resetting repeatedly will not solve it; get the boiler diagnosed properly.
Ignition / flame detection lockout
Overheat — primary temperature too high
See all Worcester Bosch fault codes.