Accepted Answer

The P0420 is for Catalyst efficiency.
A common failure is the Catalyst itself.
If you replaced the pre and post sensors, I would say the Cat can no longer preform up to spec...
A lab scope can graph the rear sensor following the front sensor with little difference, the rear should be much slower in its graph if the cat is working properly.
I can say that a VW OEM catalyst will be required. The Aftermarket cats do not have enough of the rare metals needed to convert up to the VW standards. Almost every aftermarket cat I have tried has caused the same code to return, the P0420.
I have fought the manufacturers and sellers of these cats several times, often a new customer will come in with this code, been to several different shops with no satisfaction. I tell them that they need a cat, and alot of times they get mad and I have to prove it to them, as they JUST had a cat installed by the other shops. I have a large file just on these cases to prove to them the results.
If ALL the vacuum leaks have been cured, I would suspect the cat.
Jason

Expert:	vwnewton
Pos. Feedback:	98.5 %
Accepts:	167
Answered:	8/12/2009

VW Owner,ASE Master and L1 Advanced

25 years of VW obsession turned into a job, last 15 years as a VW Tech.