There is a little magnet sender on the 450i units. It is located by your left ankle when setting on the machine. Pull that cover off and you should see the wires for it and connector if they are the same for a 450 and 500. Think there is a magnetic piece that slides inside. You can try cleaning it first. I would do that and mine would start working for a very little bit of time. Replacement cost me like $60.00, best I can remember.
