Crank, no start and code P0420

Vehicle was in working condition. The only thing that was wrong was the catalytic converter was almost due do change. (Throwing the code p0420)
Anyhow, I had turned the vehicle off, everything was okay, or so I thought. When I went to restart the Jeep, it would only crank and not actually start. Got the vehicle towed home after sitting with it for 4 days.
Disconnected the battery for 3 days and changed every single fuse. Vehicle started after 2 weeks, but, I'm not really sure how.
Drove 2 miles, shut the vehicle off, was in the store 5 minutes, tried to start the vehicle, it started. Went back home( 2 miles) turned the vehicle off. Waited about 30 minutes then started it back up, only to turn it off to show someone that it was actually "fixed." It did perfectly. I then drove to Autozone, about 5 miles from my home. Was in AutoZone around 30 minutes. When I went to start the vehicle, it would only crank, again, and not actually start. Had been doing some serious (what I thought of as) trial and error. So far, the only thing I have found has been a bad relay that was corroded with 1 of the pins being broke off. Cleaned and changed that, hoping that was the issue. Also, I have changed the spark plugs (gauged 0.043) and the coil packs.
Nothing.
Still doesn't start.
It's getting fuel, air, all that.
I'm wondering what may be the issue because I most certainly cannot afford to take the vehicle to a dealer, especially since I bought the vehicle from a person and the warranty has been done long ago. I do not have friends nor anyone I can ask for help. So, I'm now turning to you.
Just, any help is greatly appreciated.