Why do I get a 'door open' message when the door is closed on the Impulse 616?

Getting a 'door open' message when the door is closed on the Impulse 616 could mean the door switch needs to be adjusted.

The full instructions with images and diagrams are included in the PDF document. Why do I get a 'door open' message when the door is closed on the Impulse 616?

THE PROCEDURE REQUIRES THAT THE IMPULSE IS CONNECTED TO THE ELECTRICAL SUPPLY WHILE THE COVERS ARE REMOVED AND THEREFORE SHOULD ONLY BE PERFORMED BY A COMPETENT TECHNICIAN.