If it turns out to be the cat, just get a Carsound (Magnaflow) high-flow cat from ebay. I just got one for my project Celica (2.5") and it was $57 shipped. It was another $65 to have it installed at Midas.

 

I'd like to know myself.....just because I am curious too!
Anyone have a pic of the EGR on a 3.4L engine?

 

Your right, I will check out the )2 sensor first. One of the posters above mentioned that the engine light should go on if the 02 sensor is bad. Is this always the case? Thanks for the advice guys, I appreciate it.

 

No, this is not always the case.
The CEL will come on when if falls out of spec, but there is some variance in that which allows it to emit poorly until the ECU deems that the sensor is indeed bad and needs replacement. It seems like yours may be on the way (unless you just replaced it) and could allow emissions that are not passable while the sensor remains marginally in spec. The MAF sensor is another common reason why the engine can run lean so be sure to clean it will with some electrical contact cleaner as well.