Your boiler has detected an internal electrical communication error, meaning it cannot safely confirm the fan is working to clear waste gases.

Is there 240V~ between 1 on air pressure switch and 10? Check yellow cable between printed circuit board and air pressure switch. If satisfactory replace printed circuit board.

What causes this fault?

This fault occurs when the boiler's main control board loses electrical contact with the safety sensors that monitor the air flow. The most common reason is a physical break or loose connection in the wiring, or a component failure on the circuit board itself that prevents it from reading the fan's status. Essentially, the boiler's 'brain' can no longer talk to its 'lungs,' so it shuts down for safety.