Recycling soot. Probably burns once it actually gets to the cylinder, but it loves to coat the inside of everything before it gets to the cylinder. Either an expensive cleaning, or just outright replacing the air tube (normally called an intake manifold) oh that's made out of plastic. That's where those wonderful swirl valves that are actuated with a solenoid are located and that's what stops working and causes that stupid light to come on. Seen pictures of the intakes to the cylinder just coated with the stuff. Someone should have decided right then that making a clean diesel is just a stupid idea as implemented now.
Swirl valves supposedly improve low and medium response. BMW figured out how to get rid of them, it caused them so much trouble.