Okay considering that I have 163,000 miles, I figured I might as well replace the rear o2 sensor. I replaced the front couple thousand miles back. I do have a berk test pipe which I installed last year. Keep in mind I did not have any CEL's before this. So I installed the Denso sensor yesterday and today my CEL came on and the code P0420. The berk test comes with CEL fix. So what gives?

 

It's all about the exact position of the sensor. (depth, direction of holes) Just add one de-fowler and you'll be fine. The CEL fix's on TP's aren't a 100% guarantee....