PDC sensors fault on my dash

Welcome to 2CarPros. 99% of the time this is a fault in one of the ultrasonic sensors in the bumper. Those are the little round circles in the front and rear. There are tools you can use to test them or try this simple test. Get a piece of garden hose or similar that is large enough to cover most of the sensor. Then in a quiet area take the hose, leave the key on and listen to each sensor. A good sensor makes a very faint clicking sound. A bad one will be quiet.
There is also a TSB on this issue which shows it may be a software glitch.
To determine that you would need to have a dealer scan it with the BMW tool to see if the code - 0x481E4D - PMA control unit internal, internal ASIC fault - Is stored in the system.
If it is there is a software update that can fix the issue.

As for replacing the sensors, it depends on which position is bad, and if you can get the correct part.
The rear ones can be accessed from underneath the car. There are a couple plastic shields that have plastic rivets holding them. Remove the rivets, the center pulls out and then the rivet comes our, remove the shield and you can see the sensors. There are two tabs that retain them. Gently spread the clips and pull the sensor out.

The fronts depend on which trim you have, The common models have small grills at the corners that can be removed to get to the outer sensors and the center trim grill comes out for the two middle ones. If you have the sport trim package, the front bumper cover has to come off. Then you can get to the sensors.