Not all emissions tests are conducted with OBD2 link. In some relaxed regimes, OBD2 interrogation is the only component of the emissions test, and the car pass

Not all emissions tests are conducted with OBD2 link. In some relaxed regimes, OBD2 interrogation is the only component of the emissions test, and the car passes if the car determined there is no faults. I lived in a county with emissions testing and the OBD2 link was used as a pre-test before the dyno run, and to feed the VIN number into the state's computer to provide some assurance about the identity of the car being tested. Some vehicles, like my Toyota, indeed behave slightly differently if there is a diagnosis computer connected. I don't know if this applies to the OBD2 link as well as the toyota-specific connector under the hood, but it's pointed out in the factory shop manual. At any rate this knowledge has prevented me from trying the "scangauge" type of data collection device.