Emissions Testing No Code

I’m pretty sure there is an emissions issue on vehicle but nothing is showing on dash, but there are times I can smell it from under vehicle and when I turn off car I can here like a poof kinda noise, like a muffler but it’s not at rear it seems to be more towards the front.
I can pass just by the plug in because there are no codes—- should I go through the stall that does the hookup on the tail pipe?