There are two O2 sensors on our car and where you got confused is you replaced the O2 sensor on our exhaust manifold. It manages the air/fuel ratio. The code you keep getting means your cat efficiency is below the acceptable threshold, which is determined by the second O2 sensor that's at the back of your cat. Don't pay for a new cat and if you do, get an OEM replacement, not some high-flow cat. You're gonna lose power if you do. I would say just do the spark plug defouler fix that Skabone posted. It's very simple and will cost probably $5.