CEL returned after O2 sensor replace. Replace the second one and see or..?

[camhockey123]

I just recently bought my 01 Jetta VR6 GLS and about 5 days of driving, the check engine light popped up outta nowhere. It was running great (and still is), no hiccups or odd behaviors. So i had a local shop run the code and it came up P0420 which was catalytic converter efficiency below threshold bank 1. The guy said it would most likely be the first O2, so i replaced it. Then about 30 miles later it pops up with the same code again. I'm baffled and don't know if i want to replace the second O2 or save the stress of seeing another CEL and just go straight to the cat as being the issue. Thanks guys in advance!

[Scotty_2.0]

Most likely the cat is done. That code is set when the cat isn't cleaning up the exhaust enough. If the rear O2 sensor has already been changed, it's the cat that' the issue.

[hoodita]

X2 on the cat.

[sermend75]

Although I drive a VR6 12V (MK3), I've lived with the same issue for about 1.5 years now... (yes, that long...)... and I already replaced both sensors and the CAT... HOWEVER, the last thing I was told is that because I changed the CAT for a Magnaflow high performance one, the ECU can't recognize the correct flow values (or something similar) on this one and it will keep doing the same, that is, the CEL will keep showing up. I was also told that the BOSCH 4 platinum spark plugs that I had installed could be the problem, so I replaced them for NGK's OEM as well, but the light comes back after a few miles again... So I guess the only thing left to change is the new CAT that I already changed but my pipe is 2.5" so it's already non OEM... oh well... Just wanted to share my similar experiences with you on the same issue...

[camhockey123]

Sorry about the long wait, but after looking around, replacing both O2s and still having the light come on, ive come to find i either need a cat or flash my ecu. I found that the mk4s have an issue with the ecu that makes it overly sensitive to the p0420 code and triggers it i it senses too many harsh emissions due to a range of factors. Bad gas, hard driving, being a vw, anything. But thats what i found. Thanks for the help though!