01GrandV8

I’m getting a code p0420 on my grand Cherokee. It goes to start and will just stall and will start rough after about 3 times. Especially when it’s cold out it just won’t start. But after I’ve got it up and running it’ll start all day. When starting it will puff out a big cloud of white ish smoke. The coolant gauge rises all the way to Hot when idling and goes back down when driving. Not sure where to look first for the issue. Also it passed smog about a week before this started so I’m hoping it’s not a bad catalytic.

Caish

Sounds like cat material has fractured and blocking flow.
I just swapped engines in a 97 and found the same thing, plus other issues.
P0420 in and of itself won't make it run rough, but blocking exhaust flow will....