I just got back from my second Colorado AIR Program emissions test, and have a crisp air quality certificate for my "2000 Isuzu Other". Generally, the VX doesn't seem to have any problems with Colorado's requirements, coming in well below the limits. I'm not sure how the Colorado limits compare with, say, the California requirments, but I've included the actual numbers from my tests below if they help.

An enhanced test is required for gasoline powered passenger cars and light trucks 1982 and newer in the seven-county Denver metro area, which includes the counties of Broomfield, Denver, Jefferson, Boulder, Douglas, and western Arapahoe and Adams counties (west of Kiowa Creek). The enhanced inspection is performed at any one of 14 inspection stations operated by AirCare Colorado. The inspection utilizes a dynamometer which simulates actual driving conditions. The test is good for two years. Motorists are alerted to the need for a test by a notice on their registration renewal forms.

Two years ago, the VX didn't fit on the AWD dyno, and so they administered a basic 2-speed idle test. They placed an inductive RPM pickup on the hood, and clamped a probe into the tailpipe. Then, they run tests while the engine is idle and when it is being held at 2500RPM.

Test 2500RPM / IDLE
HC PPM 71.2 / 53.5 (limit 220.0)
CO% 0.32 / 0.34 (limit 1.2)
CO2% 14.9402 / 14.7917
RPM 2459 / 820 (limit 1350)

This year, they managed to shoehorn the VX onto the rollers, so I got the full test.

HC GPM = 0.1144 (limit 1.2)
CO GPM = 2.4003 (limit 15.0)
CO2 GPM = 488.732
NOx GPM = 0.1675 (limit 2.0)

They also perform a pressure test on your gas cap and visually inspect the catalytic converter, oxygen sensor, and the check engine light. Two years ago, they made sure there was a restrictor plate in the fuel filler, but didn't this year.

Finally, I don't think there is any experience more terrifying that seeing some total stranger bouncing all over the rollers in your VX. If you hoard Xanax for stressful situations, an emissions test might be a good time to break into them. On the other hand, it is sort of interesting to actually SEE the front wheels hesitate for an instant before TOD sends power to them. I'm curious what the dash lights are doing during the test.