One of my rear parking sensors is convinced there's something behind it. Are they a common failure? Are they easy to replace or fix?

this can be caused by water getting into the sensor have you or someone used a pressure washer on the car .try using the wifes hair dryer on the sensor.

bill

Not pressure washed it but I did drive through a monsoon yesterday and it rained all night. I'll see if a day in the dry helps and if not give the hair drier a go tonight (before the mrs gets home )

Mine are intermittent, so I get situations where I'm about to reverse, with nothing in front of me, and the car is telling me I'm up against a brick wall! (tend to be the front ones going off rather than the rear)

Sorts itself out either within an hour, or sometime the same day, usually.

Forced some petroleum jelly around my troublesome front one and slapped some on the back of it too, after a thorough dry out of course, only lasted a few weeks and for the hassle I went to to in order to get to the back of it I'm not doing that again. Useless dealer swears that there's nothing wrong so, I've turned the fronts down as low as they'll go so they're less of a hassle when reverse parking. Thankfully it's not that often that I park nose first but I just refer to the columbus screen if and when I do and I keep an eye on the screen if / when the front is close when pulling out from a parallel park or other tight spot - not ideal but works for me.