How a TDI engine VNT turbo works and how they fail and cause limp mode or low power

See http://www.myturbodiesel.com/wiki/limp-mode-fix-repair-troubleshooting-vw-seat-skoda-audi-tdi-engine/ for details on fixing limp mode. If the problem is carbon buildup or rust inside the turbo control ring or vanes, you can clean it out. Actuators can also be replaced for most models.