Your boiler has stopped working because it cannot properly control the fan that removes waste gases from the system.

Fan problem – tachometer signal interrupted fan connection or damaged debris in fan. You need to check and clean fan debris

What causes this fault?

This fault occurs when the boiler's computer cannot detect the speed of the fan, often due to a build-up of dirt or debris inside the fan's blades. If the fan cannot spin effectively or communicate its speed back to the system, the boiler will shut down as a safety precaution to prevent dangerous fumes from staying inside the unit.