The AkusticParkSystem (APS) on my A6 (built in 10/99) no longer works in the back. Normally a short beep sounds when you start the reverse gear. Now there is only a slightly longer quieter beeping. Almost like a peiffing. If I then approach a hindrance the APS no longer beeps at all. Can it be that just the sensors are too dirty? Or what else can it be?