Faulty rear park sensors

[SirSpanky]

Hi everyone

Hope you can help out with this: The sensors have started to misbehave and now beep when there is nothing near and then choose not to beep when there is. I've read somewhere (probably here) that they can fail, and replacing them is two bolts on the bottom of the bumper and then rooting around to un-clip them...

So the question is, is there any way of finding out which one(s) have gone without replacing all 4? Or, do i just PM PJMSpeedy asking for a whole set?

Thanks for your time!