How do states use OBD II for emission inspections?

Many states connect a computer directly to a vehicle's OBD II system. The inspector can then read "faults." In some states, this type of inspection has replaced the tailpipe emission test.
See also:
Parking the vehicle in warm weather
If your vehicle has been parked in the sun in
warm weather, opening the windows and
moonroof for several minutes before driving
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