Back up sensors not working

First thing to check is that the sensors are still wired up, make sure that the loom is not damaged or disconnected at the loom plug, start here.

Check the sensors themselves. Chock the wheels, turn the key on, put it in reverse and go back and listen to each sensor. If the system is working you will hear each sensor ticking faintly. You need to get close to hear it, cover it and the sound should change. All four need to work or the system will stay off. They fail a lot!

They are all getting power. I checked via a test light. But Steve totally nailed it! One of them is not clicking. I turned the truck to accessory with out starting it and put it in reverse. I then listened for the clicking all of the click softly except for one. So I assume that sensor is out.

While listening to them, be sure to put something over the sensor and listen for it to change the sound. No change in the sound is also a bad sensor. You can get them in sets online for a lot less than parts houses or the stealership.