A twin-turbo excessively emissions-managed diesel with EGR is a carbon build-up nightmare waiting to happen. Everything on the exhaust side (Turbos, DPF, CAT) is subject to clogging whenever non-optimal combustion is present for any reason for any extended or routine period of time. This could involve anything from short trip driving to sensor malfunctions and/or mis-calibrations and other things in between.
Then you pump it back to the intake with EGR and the viscous cycle continues and amplifies. Drive it hard and keep your fingers crossed. Fuel additives are not going to do anything for this situation. Emissions deletes and water/meth are the only true solution(s).
