I said I'd send an update - and now I don't want to, because it's confusing
I had been enduring this problem (one rear sensor erroneously triggering) consistently for the past 3 months.
Before swapping out the faulty sensor for the new (cheap) one, I did some checking:
- I disconnected the faulty sensor and ignited the car - "Auto PDC failure"
- I reconnect the faulty sensor - it triggers again (with no failure message).
- I swapped sensor 1 and sensor 2 (by position) and the faulty triggering moved accordingly (with the sensor).
- I replace the faulty sensor (in its new position) with the new one - "Auto PDC failure"
- I replace the new sensor with the old faulty sensor (in its new position) - that sensor is now fine but the one that has been moved to the original faulty position is now triggering. (go figure)
In the course of doing all this, I used contact clean on all the joints.
So, at that point, I concluded that :
a) the new cheap sensor doesn't work.
b) the original triggering might have been caused by a wiring problem at that sensor position (but the problem moved when I moved the sensor?).
I decided to bite the bullet and get it fixed professionally.
When I backed out of the garage - SILENCE!!!
No failure message - everything working as it should
I would like to again test the new cheap sensor (just to be sure it is no good - and I can leave appropriate feedback) but am reluctant to disturb things.
I have discovered that it is possible to replace any of these sensors without having to remove the whole bumper. By undoing 2-3 bolts along the bottom of the shirt, you can get your hand in to unplug the cables. The sensor fits into a plastic seat that has two long flexible posts that run up either side of the barrel of the sensor. To release the sensor, pry these away from the sensor barrel to release the retaining clips. Press the sensor from the outside and it slips out easily.