I have a 2017 Hyundai Elantra for reference. Back in December of last year, I had my engine replaced with a used one. 3 weeks after replacement, my engine light came back on that showed the code PO420. I was told that I would have to get my catalytic converter replaced. Other than the engine light, my car ran fine and nothing was out the norm until a week ago, my car started jerking.

I went back to my original mechanic shop but after checking, they said the jerking is because of the converter. I went to a second shop to get a second opinion and after they ran a full diagnostic test, they said the same thing. The issue is i went to a muffler shop and he is telling me he do not think the jerking has nothing to do with converter and that if it is something else, it will just ruin the new converter.

I also want to mention, if im on the highway, it does not jerk as much as when im driving around the city. Sometimes if i sit at a stoplight, the jerking increases and sometimes the car would pull forward but i also notice, if i put it in park while at the light, the jerking goes away just a little. Should I just replace the converter or look further to see what else could be wrong?