This code is not actually an error; it simply indicates that your boiler is currently running to provide heating for your radiators.

operating in CH mode.

What causes this fault?

This fault occurs when the boiler's main computer loses its ability to correctly monitor the electrical circuit or detects a leak of electricity to the ground. The most common reason is a component failure within the control board itself or a loose connection in the internal wiring that breaks the safety loop. In some cases, a worn-out ignition electrode can also interfere with the unit's electrical signals.