The cell is a sensitive part, which can easily be hampered. To ensure user safety, a photoelectric cell detects an obstacle when the doors are closed, and a force sensor limits tightening by the jacks.
The technician knows right away that the problem is with the doors, and where. He then deduces that the problem is not linked to a mechanism specific to the elevator, but probably comes from outside. Arrived on site, the technician will indeed find a piece of paper blocking the proper functioning of the cabin doors. 👇