If you put the car in reverse, you should be able to check the sensors are at least active by touching gently with your finger and feeling the vibration.
Mine were a bit intermittent recently.
If you remove the drivers side rear bulb cover and pull out the carpet rivet you can peel back the carpet and find the PDC unit. I unplugged mine and then reconnected and it's been OK since so I guess the contacts were a bit tarnished.