SES on, engine sputtering

[FirebirdFitz]

I had a racing exaust shop install a 3" cat back with a dynomax muffler. I've has a SES light on for about 6 months. I had it checked for free at autozone and I got a O2 sensor problem. The engione will run fine one day then sputter the next. Can I hurt the engine this way ? Also should I just pay my mechanic to fix it or is it possible for me to diagnose which sensor to replace and do it myself? Thanks. Great web site.

[hawgs]

did you get a new cat at the same time? you probably need an o2 sim. let me know if you need one.