Albert, a defective sensor can cause the problem you describe. Although the electrical connectors to all the sensors are weathertight, sometimes moisture can cause problems. MB dealers can check for trouble codes and determine which sensor is causing the problem if that's the case. The odds are that's the problem. Another possibility is minor bumper damage. I've seen many cases of sensors being slightly knocked out of their mounts by parking lot mis-haps. You have to look closely to see this. On the lighter side, I once saw a 140 that had it's bumper painted right over the sensors by a body shop that didn't know any better, but that was an easy one to diagnose!!! Good luck