I have a backup sensor (right rear) that comes on full when we've had some very wet weather. Nothing is blocking it and I wipe it off. When I put the truck in reverse it fires full (outlines red on the dash) and stays on until I put the transmission in drive again. It might do it a few times during that trip When I put it in reverse then it might stop the next time I put the truck in reverse. It seems intermittent as sometimes it doesn't do it. But, when it does it's only when it's been raining.

Interestingly my truck in under cover and when it rains it does it before I take it on the road.

Anyone have this issue and find out what it was? I've told the ford mechanic but he says he'd have to have it firing to know.

 

You could always swap it with the left one and see if the issue follows the sensor.

 

its still under warranty... right?

Just take it in and have the sensor replaced. Sounds like moisture is getting into it and faulting it out.