In my experience (CA and BC), if the test goes quickly the results
will be good.

If it is taking 2-4 minutes, things are not going as well.   I guess
the machines are calibrated to quickly pass clean vehicles and give
flaky vehicles more time to get a stable reading.

>aircare colorado runs the tests on rollers, running vehicles through
>a series of speed changes.  sometimes the tests are pretty quick,
>sometimes they are running it for several minutes.