It may have a magnet sensor on the step. Follow the wire by the step. Ours has a small Radio Shack magnet sensor. It becomes loose as the 3m tape they used to attach it dries out. Plastic to powdercoated metal is a hard attachment.
Also I have found ones where the magnet fell out of the plastic holder. Small round magnet 1/4 inch diameter, maybe 1" long....if you see it stuck to the steps somewhere, there's your problem.