I have a 2003(53) plate Mercedes E270 Automatic saloon which has factory fitted front & rear parking sensors. When I first put the key in the ignition both front and rear red lights and the high pitched note come on. When I get set off they go off as normal but under braking the Front parking red warning lights come on with the high pitched note. I have tried washing and cleaning around the sensors and the car has not been bumped etc

 

sometimes an installed front plate will cause that. also if they are just slightly pushed in or bumped they will act crazy like that. perfect positioning is critical for them to work correctly.

 

Many thanks chassis221 will have a look and see if they've been pushed in or moved