The P0420 -Catalytic converter system below threshold code- may be Bank1, front O2 sensor, but if youve scanned it already then you'ed know if that one was bad.
Any other codes besides P0420?
Its a 2001 12v. Ive already replaced the rear o2 and that didnt solve it. I also replaced the cat and it didnt fix the problem. Ive also replaced the plugs and wires. I also heard about a computer flash for the early vr6s for this same issue, but I called VW and they said that the flash was applied to my car already. Is there anything else I should be looking at before I drive this thing into a wall?
Cat efficiency codes are either the post-cat o2 sensor or the cat itself. To know for sure you'll need a VAG-COM. Think there's a link in the mk4 FAQ about testing your car and post-cat o2 sensor.