What are Emissions Tests?

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  • Written By: Jessica Hobby
  • Edited By: Heather Bailey
  • Last Modified Date: 13 September 2019
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Emissions tests are procedures used to gauge how much pollution is emitted into the air by a motorized vehicle. Most states require that a vehicle is tested every two years. The primary goal of an emissions test is to reduce pollution caused by vehicles that release hydrocarbons and other pollutants in the air. Vehicles that are tested and fall below National Ambient Air Quality Standards (NAAQS) are required to be fixed before their owners may operate them legally. Vehicle emissions testing requirements have changed over the years.

Originally enacted in 1970, the Clean Air Act was amended in 1990 and mandated that areas with substandard pollution rates and areas with more than 100,000 people institute vehicle emissions tests. Subsequently, the Environmental Protection Agency (EPA), which is responsible for administering the Clean Air Act, was required to create standard operating procedures for emissions testing. After issuing its standards in late 1992, all areas that required emissions testing were forced to use a 240 second inspection and maintenance test (I/M 240).


The I/M 240 test identifies a vehicle’s pollution problems by measuring the amount of hydrocarbons, carbon dioxide and nitrogen oxides while a vehicle goes through a simulated driving cycle. Drivers must take their vehicle to a testing station where they are instructed to drive onto a device which is similar to a treadmill for automobiles called a dynamometer. Pollution is gathered from the tailpipe of a vehicle while it is idling, accelerating, cruising and decelerating.

Vehicle emissions tests changed again in 1995 when the United States Congress passed the National Highway System Designation Act. The act prohibited the EPA from requiring the I/M 240 test. As long as the areas of concerned met NAAQS, states were given the freedom to use whatever technology they preferred to test vehicles. There are different types of emissions tests administered based on the age of the vehicle.

As of August 2009, the most up-to-date version of the I/M 240 test is the I/M 93 test, which is primarily used on vehicles built between 1981 and 1995. Vehicles built in 1996 or later usually receive a second generation on-board diagnostics test (OBDII) where information is taken from the vehicle’s computer system. In some states, vehicles older than 1981 require a single speed idle exhaust test, where a tailpipe probe measures pollutants released in the exhaust.


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