hmmmmmmm!

It could possibly be a code 67 "Catalyst System". You could be right as well something could be up with the O2 sensor.

You could try to reset the CEL but I have a feeling that wont work.

What kind of high flow cat did you purchase?
Do you know how to jump the ECU to retrieve the check engine light code?

If it's a code 67 good luck trying to get rid of it. :chuckles:
Code 67 is a bitch and as long as you have that high flow cat on, that code is going to stay.

You could very well have trouble passing emissions. Welcome to the world of OBD2 and the problems that come with modifying the emissions system.