The owner's manual on my car describes your parktronic behavior as a malfuction warning. The dealer service manager here tells me that usually it is a plugged sensor (not cleaned properly) or a sensor which has been hit hard and rendered inoperable.

Cleaning or changing the bad sensor should cure the problem. If not, then the ECU of the parktronic system should be changed. Before doing this, check for improperly attached or cut cables inside the bumper assembly (maybe from a previous fender bender).

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]