Code P0420 car stalls at idle, car moves very slowly

i was going down the highway, and the check engine light came on then about 30 seconds later the car stopped accelerating and came to a crawl. upon stopping the vehicle stopped running on the side of the road. I have the downstream air sensors removed and the flexible exhaust tube removed so I can see the back end of the cat right now. it has some darker spots on the cat that I can see and the sensors are both covered in soot. It reads the p0420 code immediately after the vehicle is started and idles really hard then shuts off pretty quickly. Is there a way to tell if the cat is bad and to check the o2 sensors?