back up sensors not working.

[brndfrb]

So my back up sensors are not working on 2003 F 350. You can turn them on from the dash button but as soon as you put the truck in reverse they switch off. Any ideas?

[NYC F-350]

One (or more) of the sensors is junk.
Set the e-brake, stick it in reverse. Listen at each sensor. The sensors should emit a quick succession of clicks. If it clicks slower than the other sensors or does not click at all, replace the sensor.