Your boiler has detected an internal electrical communication fault which is preventing it from starting up safely.

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 the ability to communicate with the sensors that monitor air flow and safety. The most common reason is a failure of the internal electrical circuits on the main control board or a loose connection in the wiring that links it to the air pressure switch. Because the boiler cannot confirm it is safe to vent exhaust gases, it prevents the burner from firing as a safety precaution.