We just had a similar problem with the same brand step. It turns out the magnet which actuates the small sensor fell off and as a result the step wouldn't work. The sensor is mounted 6" off the floor on the base of the door frame. If you look at the screen door, you'll see a magnet or where the magnet used to be if it fell off. As soon as I discovered the magnet on the ground, I found where it fell off and quickly glued it back to the screen door frame and all is well again.
One more point I thought I would make. Clean and lube the step components and you should have a working step for a while. Otherwise it will rust inside extension arms.
Thanks texnet for the link to the manual, I added that to my HD library.
Carol & Mitch roaming around in a new 2013 Thor Tuscany 40EX