normally prox sensors will have a light to indicate when it is reading metal. if metal is over the prox it should be lit. there should be a test that you can run through the maintenance mode of the unit that will let you know if the truck sees that the prox is functioning correctly. my guess though is you should be testing the potentiometers on the unit. i am assuming you are working on a turret truck? check the rotate position pot and the traverse pot, these have test you can access through maintenance mode as well. if you need to get into this mode google search crowns phone number and enter the last four digits as the password. that is unless it has been changed. after that, that is all i have you need a access 123 manual to navigate the menu