imola,
turn on your pdc first, while parked of course, have the pdc light blinking and go outside put your ear right next to the sensor, check each one of them, if the sensors work, you'll still hear a little clicking sound very faint sound, that'd be first step. if you hear sound from all of them, then it might be the wiring behind dash underneath steering wheel, maybe something pinched in there when you had any wiring work done, i've had same thing happen awhile ago