CEL came on a few months ago. Ran the code ending up with P0420. Replaced cat with a magnaflow. (this might be the problem) *Note* entire exhaust system is stock minus that cat. Light stayed off until now. The light came back on ran the code.. same problem. Changed the o2 sensors. Disconnected battery waited 10 min; light is still on. I live in Nebraska and there is no 93 ocatane so i'm FORCED to use 91 with an octane additive. I have read on here that I could of got ahold of some bad fuel maybe? Could it be an exhaust leak? Vac leak? I'm concerned because i'm only getting 220 miles to a tank. If anyone could be of any help i would appreciate it.

 

It's the cat, most aftermarket cats aren't going to keep a cel away, there's a reason why the OEM ones cost so much, they have more of the catalyst.

 

I have the same issue with my 2000SH. I installed an Eastern Catalytic OE style direct fit converter that I bought from Amazon in April 2010. I have also replaced both O2 sensors with NTK (NGK) ones. I am leaning toward a leaking flexpipe triggering the CEL.

 

maybe you should have your cat checked? does it feel like you have lost hp if you got some bad gas you should be running really bad or not at all.

 

On a side note, if your car is (mostly) stock, you should be fine on 91 octane; you don't need to waste money on the octane boosters. A leak would probably trigger a code for the primary o2 sensor first, not the cat from what I understand from the manual. Did the CEL stay on after you reset the ecu or did it turn off and come back on after driving it for a little while? It would take two driving cycles for the CEL to come on for an emissions problem; it wouldn't happen right away.

 

does your car ever sputter?