Parking aid problem

My parking aid is not. Working on my2003 Expedition. At first, I was getting the "Check back up aid" message with a solid tone when I put it in reverse, now when I put it in reverse, I get no message, no tone, nothing at all. Anyone have any ideas?

Check thew alignment of the sensors, then see if the wiring connectors are good going to them, than check the switch, than tha fuse. Or do reverse order.