Q: My car won't start. Diagnostic test read P0420 catalyst efficiency below normal (bank 1). Could this be why it won't start?

My car cut off while I was driving it. I got a jump & it started back up then cut off after a couple of minutes. I jumped it again & it started. I immediately drove it home. I was about four blocks away. The car barely made it. The engine would rev up, stop, rev up, stop. It wouldn't go faster than 10mph. The steering was also stiff while this was happening. It cut off again as soon as I got home. I was able to start it back up after about 30 minutes without jumping it. I let it run for a couple of minutes but I didn't drive it. The next mourning it started & I attempted to take it to the shop. I stopped at the store about 2 miles away from my house & it cut off again. I got a jump & it started. While it was running, I disconnected the negative cable to the battery & it cut off immediately. I jumped it again & it started but kept cutting off before I could drive it so I had it towed home. It's been 2 two days & it won't start. I haven't jumped it again. It does have gas.

My car has 145000 miles.
My car has an automatic transmission.