My 2002 Jetta Wagon 2.0 keeps throwing the P0420 code. I know there are many threads on this subject, but this is unique. I got a brand new cat under warranty last August so unless I got a dud, it is not that. I replaced the rear o2 sensor with an OEM from 1stvwparts.com so again, unless it it s dud, it's not that either.

While looking at the measuring blocks, the post o2 sensor shows 000 until the cat warms up and then it reads the 110 it is supposed to. The pre o2 heater resistance is like 78 ohms, where the post cat o2 is .1K...which is in limits according to the VagCom...weird thing is that sometimes the rear o2 sensor resistance doesn't register at first (did this with the old sensor too).

The self tests in basic settings all pass. Both sensors check OK and the pre cat o2 sensor's aging reads 1.5xx to 1.99. I think that 1.99 is the value of a new sensor. The cat test, however, sometimes passes and sometimes does not. Most of the time it passes, but every once in a while it fails.

Any ideas? Something I am not quite sure about is if there is a seal on the o2 sensor that should have been replaced with it. It is plenty tight, but is that enough to seal it? Thanks.