The PDC enables as you engage reverse and you are met by a long tone and then no PDC function, indicating there is a fault.
To check the sensors the engine does not have to be running, just but the ignition to second position so the dash lights up and press the PDC button. The long tone will sound. You can now get out to check the sensors.
Put your ear close to each one in turn, you should hear a very faint ticking sound. Any that don't are bad or have a wiring fault. In my experience the sensors do just go faulty, that is ones that look pristine and have clean electrical connectors can just stop working.
Order a replacement sensor. 66206989069 from ebay, £10 delivered
I had a search of the web about removing and refitting the sensors and didn't find a lot of encouragement, some indication of having to remove the bumper.
I can tell you on a model with unpainted bumpers it is possible to replace the rear inner sensors without having to take the bumper off because this is the one I did.
To replace the RHS inner sensor.
Remove the tow hitch cover.
Reach you hand in the gap to the back of the sensor.
Feel for the plug, squeeze and remove the electrical connector, this is in line with the body of the sensor but at the top.
The sensor can now be removed, tricky working blind but quite doable.
The sensor is held into the bumper receptacle with two fingers. Splay the two fingers downward and pull the sensor out. Theses fingers hold onto plastic pips on the sensor holding it in place. The fingers are quite flexible, my tip is to grip the body in the palm of your hand, splay one finger downward and pull on the sensor at the same time, then while still maintaining a pulling force on the sensor (this keeps the pip out of the finger ), splay the other finger. The sensor should then start to pull out from the rear.
Slide the new one in paying attention to the orientation.
My faulty sensor. New one in and all working ok.