 More on RSS Sensor Systems
After posting my previous message, I stumbled on this article on line. Thought I'd pass it along:
Bumper Sensors
The sensors in bumper sensor systems are visible by the circular sensor face in the bumper cover like on the rear of the 2005 Cadillac XLR. One of several models that have a similar sensor array in the front bumper is the 2005 Lexus ES sedan. There may be up to four sensors in each bumper, which are connected via a wiring harness to a dedicated control module that communicates to the main body computer. The sensors are either all ultrasonic, or a combination of ultrasonic and radar. The system does not have to be active at all times. There is typically a switch in the instrument panel to turn this system ON and OFF, with a text message or LED lamp indicating the system status.
In the case of the rear parking assist system, which is the most common, the system will only sense when the reverse gear is selected. The beeping noise, indicating the bumper is getting close to an object, usually comes from a dedicated rear speaker. The sound may intensify as the vehicle gets nearer to an object, and turn to a constant tone when the object is extremely close, about 30 cm (12 inches).
What can go wrong with these systems? A misaligned or loose fitting bumper or bumper cover can affect system operation. The sensor mountings are separate parts that are inserted in holes in the bumper. This mounting can also loosen or get misaligned. Visual inspection may reveal a loose or out-of-position sensor. Ford Motor Company has a procedure for checking the alignment of the sensor array on the Ford Expedition and Lincoln Navigator. There is a test for proper sensor height and another test for ensuring the sensors detect an object in five specific distances from the bumper.
The sensors can get dirty or iced over, which can interrupt the signal communication. Ford recommends washing the bumper and sensors with high-pressure water to determine if this is the problem. There may also be a problem with the finish on the faces of the sensors. Some sensors are not painted, but others require a finish on whatever is visible on the bumper cover.
For the finished bumper sensors on the 2005 Cadillac XLR, General Motors cautions to not exceed 6 mils of coating thickness on the face of the sensors. Along with that caution, there is a recommendation to not refinish previously finished sensors, since excess build-up will cause the sensor to be inoperative. There are also no recommendations for methods of removing excessive finish on the face of the sensors.
If visual inspection doesn’t show a problem, a diagnostic check may. Electronic faults will usually trigger a fault code, which can be identified, and diagnosed using a scan tool and following the troubleshooting flowchart procedures in the service information. The diagnosis may lead to a fault, such as a cut or pinched wire, which can be repaired. The diagnosis may also indicate a control module or sensor malfunction, which usually requires replacement of one or the other, or both. If the testing shows a problem with only one sensor, one sensor can usually be replaced. It’s typically not necessary to replace all of the sensors.
Based on previous posts, there may be an electronic fault code which can be traced. I'm going to do some more looking around on my vehicle. Best of luck with yours - Let me know if you find anything. | 
