Went a smog flogging last month, and after replacing the O2 sensor, tuneup and catalytic converter, She passed well. Well not as well as I'd hoped but well enough to go from gross polluter to good dog.
Long story short. In the aftermath of replacing the belts, the car quieted down and i could hear an exhaust leak. Sure as sunshine I'd lost two of the three nuts on my exhaust flange, and I'd bet you lunch that's where my excess air was coming from.
thought you'd like to know
Paul