My HX is a 49-state model, but from what I can tell there are no physical differences between CARB and other models sold in the US. The ECU may hold the key. If you have a scangauge, does it go to 9999 MPG very often? Mine does this when I hit the throttle sweet spot, so if you don't get this in regular driving, the CPU is probably different.
Lean burn does pollute more while burning less fuel AFAIK, but that probably wouldn't cause you to fail an emissions test... then again...
