Your boiler has detected an internal communication error between its main control boards, meaning it cannot operate safely and has shut down.

BMM fault.

What causes this fault?

This fault occurs when the two internal computers inside your boiler stop talking to each other. It is usually caused by a physical failure of the electronic circuits or a loose connection in the internal wiring, meaning the boiler can no longer verify it is operating safely. Because the system can't confirm vital safety checks, it shuts down completely as a protective measure.