ODB code P0420
 

I have a 2001 volkswagon golf 1.8L turbo engine, auto with around 121,000 miles. I keep getting a P0420 code. I have changed the catalytic convertor, upsteam and down stream O2 sensors and even had it at a volkswagon dealer who doesn't show any problems in the car. I even got an inpection sticker. Any suggestions on what I may be missing?