MAKE SURE THAT THE NEW SENSORS ARE REINFORCED WHERE THEY ATTACH TO THE WIRE HOLDING BRACKETS. MERCEDES UPDATED THEM ALONG TIME AGO. THE REAR SENSOR USUALLY DOES NOT GO BAD BUT IT IS POSSIBLE. IT IS VERY EASY TO REPLACE. YOU CAN TEST THEM WITH AN OHM METER. WIGGLE THE WIRES WHILE TESTING TO SEE IF THERE IS AN OPEN.