In the right margin of the sliding panel, there are two magnets: one in top right corner and one in lower right corner. When the magnet appears behind the electronic board, the board senses it and stops the door at that location. If you have a missing magnet, first look to see if it is sliding around on the panel or look behind the battery box. They jump like fleas but (almost?) never fall off the door assembly. So you probably don't ever need to buy a magnet.