if dpf filter then its due to the ecu has been programmed by the factory to keep running as long as possible and not to throw a eml to go in to restricted use ie limp mode as just before a healthy dpf is regen'd its a good 80% full of soot carbons after the regn its all gone back to free flow again until next time, but if dont clear its self it just gets slower and slower until finally will get dpf light on dash,, the trouble is a vnt turbo was never really designed to work with a filter in exhaust backing up carbon in to its nozzles so this reduces its boost by not spinning the shaft inside fast enough because its working against a high pressure around the exhaust side of the impeller,, also the egr valves can get stuck slightly ope or the seat it sits in will leak due to carbon build up from back pressure from a partially blocked dpf filter and also bleed pressure away, the whole system must be treated as one part of the car when cleaning repairing as all parts must work together to keep each other working correctly as such, this i believe is the knee jerk reactions the car makers were forced to do when all the governments decided that cars are the reason for most of the pollution,,, hmm but still thousands of planes and ships run around with no restrictions pretty much. but hayho we are where we are and joys of diesels, i own a diesel always have but they come with some must do jobs mileage related to stay healthy , ie filter change/75k additive refill if have one /45k once a week a 3000 revs run for 20 miles to kick the passive regen going on a healthy system ie without any fault codes,, even a faulty glow plug will block it all up,, this all comes form someone who has done a number of courses on these systems over the years,,, if were we have filter gone must keep the same locking exhaust any welds must be hidden or lock like factory ones and have egr gone at same time need ecu remapping to do map out, watch mpg go up by 10mpg on most things, can drive around at 1500 revs for the rest of ots life and will not do it filter and wont damage the timing chain as mush as well. down side its mot failure if they find it, thats said its only a visual check and very impossible to detect if done correctly, ironic thing is after this the car will never smoke never waste diesel having to rev so high to keep its self clean,,