i was told that the PDC give the error sound even if only 1 sensor is spoil, in my case it was the broken wire, the bloody mouse got to the wire near the front bumper, i reconnect the wire and PDC is back working, so check the wire.
To find out if your PDC is working. switch on the ignition do not start the engine put to reverse gear and put your ear and listen to the sensors up close. the ones that are working will have a continuos ticking sound. if there is no ticking sound then it's faulty. to reconfirm connect to the one that is working.