The OBD II test is both quicker and more effective in identifying polluting
vehicles than a tailpipe test.
It also provides information to a licensed technician about what repairs are needed.
Whenever possible, vehicles are given an OBD II test rather than a tailpipe test,
though there are some OBD II equipped vehicles, especially some import vehicles,
that must be given a tailpipe test because of design problems.
Vehicles that are given an OBD II emissions test will not have the tailpipe gases
sampled because OBD II vehicles have already been self-testing all their emission
control systems under different driving conditions. The vehicle tests itself and
the test station merely records the results of the vehicle self-tests!
Vehicles will pass or fail the emissions inspection based on whether the "Check Engine Light",
also called the Malfunction Indicator Light (MIL), is working and whether or not there are Diagnostic
Trouble Codes (DTCs) stored in the computer's memory.
