[GoLowDrew]

I'm a Cobalt owner for two weeks and only looked under the hood once, so excuse me for not knowing. Maybe (you) other can chime in. The Cobalt should have at least two O2 sensors. I see the first one is on top. One of them is the one that measures (controls) the air/fuel mixture. As long as that one is not messed with, your engine should be fine. I also recall the header and first cat is seperate, so replacing the header should not affect emission, unless you do the "race" setup, where you do remove that cat. Now if I could only recall where that first O2 sensor is, before or after the cat. I guess I need to look over the car this weekend.