Agreed, about two weeks after replacing a faulty sensor on mine a second failed so I just replaced the remaining three on second visitby the time you have messed around , struggled to get at them in situ, and just generally got issed off with the job, it would have been just as quick and a lot less hassle to remove the bumper and while its off check the others , its sods law that as soon as you do one, another will play up, a bit of preventative maintainance never hurts.
I know you need to whip out some of the rear load area trim , but its not that bigger task.
good luck.