Importance of emissions tests on cars

States require exhaust and emissions testing in well-traveled metropolitan areas to identify motor vehicles that are responsible for releasing air pollutants above federal health standards. States typically require motorists to undergo vehicle exhaust and emissions testing every other year in order to register their vehicles. If a vehicle fails the test it must be repaired and retested before it can be registered.

Motor vehicles pollute through the emission of carbon monoxide and explosive organic compounds. Because of this the federal government passed the Clean Air Act which outlines standards that all areas must meet.

Exhaust and emissions testing ensures that a car is not polluting the air excessively. California’s smog check program removes 400 tons of pollutants from the air each day. 10 percent of vehicles contribute to half of all air pollution. Vehicle exhaust and emissions can cause ozone buildup commonly known as smog. Newer vehicles typically have more sophisticated emissions control equipment but they still need to be tested to ensure that this equipment is working properly. Older vehicles do not have such equipment and may be emitting pollutants that don't meet health standards.

If their vehicle needs a test, individuals are notified when they receive their registration renewal notice. Testing costs about $15 depending on the state and car owners must take their vehicles to a garage with emission testing equipment. Typically two different types of tests are performed at garages. The first is an on-board diagnostic test that is usually performed on newer-model vehicles, light-duty trucks and sport utility vehicles. An on-board diagnostic test consists of connecting a vehicle's onboard computer to a scan tool to determine if the equipment is in proper working order. The second test is a single-speed idle exhaust test and gas cap pressure test. This test is performed on heavy-duty trucks. The carbon monoxide and hydrocarbon pollutants from the vehicle exhaust are measured as the vehicle idles. The gas cap test checks the cap's seal to ensure that fumes aren't released into the air. For a diesel vehicle exhaust may be tested with a snap acceleration test that simulates on-the-road acceleration.

If a vehicle fails the test, it means it's not running properly and is polluting the air. Individuals whose vehicles fail usually have 30 days to have the emission problem fixed. After this, they must have their vehicles retested. In most states, the recheck is free if it's done within 30 days, but individuals who wait longer than this must pay again.