There are many possible causes, knowing the error codes would help narrow it down. It could be vacuum line problems, sticking turbos, bad solenoid, bad wiring, you have to narrow it down or else you can waste a lot of time and money chasing the problem.
VW TDI forum, Audi, Porsche, and Chevy Cruze Diesel forum
A forum community dedicated to Audi, Volkswagen, Porsche, and all turbo diesel owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, TDI, maintenance, and more!