Air source, exhaust leak
 

Sort of an oddball question/problem.

I noticed that whenever I run the fan for climate control to pull in fresh air, and I'm at a stop, I can usually start smelling exhaust fumes coming into the car. I thought at first this was a side effect of the midpipe, but I've been running with the cat lately and I noticed it too... just not as severe. This is anecdotal, but it may also get worse running the defroster.

Recirculation mode doesn't have this issue. Where does the air come in for "fresh" mode? I haven't gotten too up close and personal, but I haven't noticed an exhaust leak near the headers or anything, and I'm not throwing any CELs.

The air should be coming in at the same spot for all cars, at the top of the hood right below your windshield.

What kind of gasket are you using at the header > midpipe connection? I noticed a leak on mine when using the stock ring gasket and had to switch to a carbon crush gasket. The leak only happened with my midpipe, the stock pipe was fine with the ring gasket. You would know pretty easily if you had an exhaust leak happening just from the sound while driving by walls or other cars. If it seems excessive when something is right next to you then you probably have a leak.

The best way to see if you have a leak is to just get under there and feel for it with the car running. If you cannot feel it with your hand you can use a feather or something similar to see if any air is flowing out around that connection.

Yeah I'm using the stock gasket, but I'm running with the cat on now and I still notice it to some degree, just not as bad. I thought it was just the midpipe combined with running rich, but perhaps it's something else.

Wouldn't I throw a code if I had a noticeable leak before the O2 sensor? I think the connection is pretty solid, but if and when I throw my midpipe back on maybe I'll pick up a different gasket.

I definitely have a leak post O2, right where the muffler meets the intermediate pipe, but I wouldn't think that would make it the whole way up to the front of the vehicle.