My cat was plugged, makin all kinds of noise. I took it off and hollowed it out. Been that way for 3 years. No codes. Well for the cat or emission system. It even passed the emmision test with flying colors. I'm guessing your one of the sensors is shot and or your cat is shot. I'd start with a new o2 sensor.

 

I don't want to replace it if it isn't bad though. Haha.
It hasn't been on for a few days.