As title - any idea as to cause? Can't see any external damage. It's the one sensor on the drivers side of the back bumper. Sensors were optional extra on new car so factory fitted. Is it still under warranty?

Is it clean?

Could have fallen off and be inside the bumper.

As title - any idea as to cause? Can't see any external damage. It's the one sensor on the drivers side of the back bumper. Sensors were optional extra on new car so factory fitted. Is it still under warranty?

I had this problem, found it to be water getting in the sensor.Try blowing out hte water, or with a vac sucking it out.Then seal with some vaseline.I did this and it never came back (about 4 weeks ago).

It is clean (very) but BOBW you're right, it was water, gave it a good suck and now back to normal. I'll have to get some vaseline and finish the job off later

Mine do this on the odd occasion and just wondered if people thought some silicone spay might be more effective than Vaseline?

I packed with silicone grease and has held for > 1 year.