Lemme think... oh yea. The good ol' EGR. Its like automotive engineering designers know they have all this hot exhaust gas and ought to do something with it. So, they started to use it to help emissions way back in the 60s. And its been with us ever since. Diesels never had emission systems, but they do now, big time. Your 2009 TDI is an example of a workable way to keep a diesel from having a lot of objectionable emissions, its a friend of the earth, me too! Beijing today looks just like LA in the late 60's, choking on smog.
At any rate, your car with 150k likely has a balky or just plain non-functional EGR valve. Change it out for a new one. Good bye P2457.
Lots of hot rod guys, God bless their hearts, have been removing or blanking or otherwise deviating from the manufacturers egr design for decades, and will continue to do so for decades to come, and not just with the egr.
BUT today's TDI, your TDI, includes an element of exhaust cooling which those pesky engineering designers somehow utilize and include in the control algorithm to keep your car clean and efficient. So, messing with the egr messes with exhaust gas temperature; inadvisable for the average motorist. The hot rod guys know that their messing around is why they have a fire extinguisher in the rear driver side footwell, right?!