As of July, 2011, 31 states in the United States require some form of emissions testing of motor vehicles as a requirement for registration. These inspections may be required annually or biannually and some states restrict smog testing to residents of certain counties, usually urban areas. Acceptable standards for the various constituents of automotive emissions vary widely by state as do options and consequences for when a vehicle fails its smog test.
Currently, California has the most comprehensive and most strict emissions standards for motor vehicles in the United States. In many states, off-line pre-testing of a vehicle before its official smog test is permitted. Going into your official smog test with necessary repairs already made, or at least armed with the advance knowledge of what the likely outcome of the test will be, has several advantages.