Your boiler is currently running in a special engineer-only testing mode used to check combustion levels, meaning it is not in its normal operating state.

CO function active

What causes this fault?

This status is triggered when the selector dial has been turned to the 'CO' position, which is a dedicated mode for engineers to test if the gas is burning safely. It usually occurs because the dial was accidentally left in this position after a service or because the knob has been moved during cleaning. It is not an internal component failure, but rather the boiler obeying a specific manual command.