Your boiler has stopped working because the pipe that lets exhaust gases out is blocked or the internal sensor that checks for airflow is clogged with dirt.

Debris in flue system, Debris in flue venturi, Boiler or flue system has developed a fault (check operation of fan, flue venturi, analogue pressure switch, flue system)

What causes this fault?

This fault occurs when the boiler's safety sensors detect that harmful exhaust gases are not being cleared from the system correctly. The most common cause is a buildup of dust or debris inside the small tubes that monitor airflow, or a mechanical failure in the fan that pushes the gases outside.