Your boiler has stopped working because the fan that clears away waste gases is not spinning at the correct speed or its signal isn't being picked up by the system.

Fan tacho signal fault

What causes this fault?

This fault occurs when the boiler's internal computer tries to spin the fan but doesn't receive the correct feedback signal to confirm it is moving safely. The most common reason is that the fan motor has physically burnt out or the delicate electronic sensor inside the fan has failed, preventing the boiler from safely venting waste gases. In some cases, it can also be caused by a loose connection or a fault on the main control board failing to process the signal.