Your boiler has stopped working because its internal computer and burner control system are failing to communicate correctly.

BMM fault.

What causes this fault?

This fault occurs when the boiler's main control board loses its digital connection with the burner management module, which acts as the 'brain' for the firing process. It is typically caused by a hardware failure within the internal circuitry or a loose connection in the wiring harness that links these two critical components. Without this constant data flow, the boiler cannot safely coordinate the ignition and heating process.