If the MPB isn't to its zero position, the LED for MPB will shine red on the main PCB. 
The LED for "OPEN NEXT/SCO CLOSE" and "ALARM" will shine green and red since those functions also will be activated.

If the MPB is to its zero position but still doesn't work, the sensor/switch is broken.

Additional video

Recommended spare parts

- If the MPB bracket itself or the Micro switch are faulty, below parts can be ordered (please note that there is one left and one right version for the MPB).