Before July 2005, when you took your vehicle to get it's emissions test done, they did so through the tailpipe and testing the exhaust. Now, a lot (if not all) of the places use a diagnostic computer that they simply hook up to your OBDII diagnostic port.

Unfortunately, my car 'should' be OBDII but because I've put in hondata (which requires an OBDI ecu) I fail the new diagnostic test.

My car runs well with the hondata so it'll pass the old tailpipe snif test if I can find a place that still does that test.