Your boiler has detected an internal communication error or hardware fault within its main control system, preventing it from firing up safely.

System fault burner control unit

What causes this fault?

This fault occurs when the boiler's internal computer and the burner control system stop talking to each other reliably. The most common reason is a electronic failure on the main circuit board or a loose connection in the internal wiring, which means the boiler can no longer safely regulate the flame.