W204 rear parking sensors duff?

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Active Member
Jul 31, 2015
w123 240TD
W204 2011 diesel.
Suddenly my rear parking sensors seem dodgy.
When I put the car in reverse, the rear sensor lights showed two red bars each side but did not detect anything.
A while later this changed to a single white bar each side when in reverse, neutral or drive.
Carsoft shows no faults. There have been no bumps and the only thing new is that I had the A/C fluid changed today but I doubt that is germane. I did drive today with a bike rack on which does confuse the sensors but unless there is some kind of reset needed I don't see what could have happened.

Any clues?


I posted this ins pite of there being some threads on this issue but none lead to any conclusions as far as I can see
OK, digging deeper in Carsoft I find a historic fault on Parktronic Sensor 8 saying there is a malfunction and 'mechanical setup is not OK'.
Number 8 is the one at the rear, inboard from the bumper corner. There is no damage to it but the message is intriguing. What does it mean by 'mechanical setup'?
I can find nothing in WIS though.

Any ideas?

Here's a thought.
I found documents from the US showing that the Parktronic system can get confused by the licence plate being bent or mounted with a slightly large gap. This would cause a fault believe it or not.
I drove my car with a bike rack attached to the towbar and I think this might have triggered a fault which, since I haven't cleared it, is stopping the system from working. I will check this tomorrow by clearing the fault and let you know
Clear the codes and all works fine.
This tells me that the Parktronic software checks the sensors AND checks the historic codes (which it should not) and as a result it shows an error and doesn't work.
I will post this separately since this issue seems to cause many problems when it is simple to solve

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