To check the system, turn on ignition (do not start engine) place car in reverse and then put your ear right up to the sensors, you should here a clicking noise, it is really faint so you must put your ear to the sensor, if you find a sensor with no clicking noise then that is the one that will need changing, if all of them do not make a noise then probably a module but get the garage to check with their software to make sure. I have just done this and found my outer near side was faulty. Luckily mine was under warranty which my dealer gave me when I brought the car, if it hadn't been it was over £400 to get my Jag garage to replace as the rear number has to be removed and believe me there is no other way but to remove the bumper. Hope this helps.