Egr pushes crud back through the engine for a cleaner burn problem is this gunks everything up, I black tar carbon. The dpf also gets this gunk and on short journeys there not enough heat to regen, so they added as blue which is meant to clean the dpf.
Problem after problem, expensive to fix
On the older 1.9tdi VW engine you could just blank it off, modern day cars are so sensitive they throw a fault
Not you can have a dpf removed and the system remapped, same with removing the ad blue but these are not failures.