I just got back from a long road trip with my C320. I really like the auto wiper option. It works very well. I did notice that in conditions where it isn't wet enough to self clear road grime like salt off of the windshield, the sensor will cause the wipers to continue wiping. Using windshield washer fluid clears the grime from the windshield/sensor and the wipers return to normal sensor operation. In my opinion, a very accurate sensor. This may sound like a pretty basic question but have you tried cleaning the sensor area with window cleaner?