94 dakota 3.9 v-6

Hello I just purchased a 1994 dodge dakota. How depandable are these trucks. The milage reads 147000 but the engine was replaced about 1yr ago and now has about 15000 miles on it. I took the truck to get it inspected and it failed the emissions test it had a high Nox reading. The tech said that it could be the catalitic converter or the egr system. One thing that I noticed was that the pvc valve would rattle while idling so I replaced it and it still does the same thing. (rattles). The problem is when I drive the truck for a distance and cut it off it has a hard time trying to restart. Could this be part of the problem. when it is cold it starts right up. I had the fuel pump tested and they tell that that was not the problem. They say that the truck has a hard time breathing what do I need to do any help would be much helpful
Thanks.

High NOx means your cylinder head temps are way high. The mechanic was right, you have a problem with your EGR system or fuel delivery. High NOx is the result of a lean mixture. I would start first by checking the EGR system.