W204 rear parking sensors duff?

[sceh]

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?

Thanks

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

 

[sceh]

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?

thanks

 

[sceh]

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

 

[sceh]

BINGO!
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