Air Care Colorado

1981 and Older (2-speed idle) Test

The two-speed idle test is performed on 1981-and-older gasoline-powered passenger cars and light duty trucks, and on 1982 and newer gasoline-powered, heavy-duty vehicles with a gross vehicle weight (GVW) greater than 8,500 lbs. The idle test uses a tachometer and a probe to monitor idle emissions. This test analyzes the carbon monoxide and hydrocarbon emissions, but not oxides of nitrogen (NOx). There is no driving simulation.

Test Process

A probe is placed in the tailpipe to measure pollutants during the test. Pollutants are measured in parts per million (ppm) and percent (%).

A contact tachometer sensor is clipped to the spark plug wires under the hood, or if the spark plugs are inaccessible a non-contact tachometer is placed on the hood of the vehicle to measure RPM (engine speed). An inspector starts the engine and lets it idle for 30 seconds, then runs the engine at 2500 RPM for 30 seconds, and finally lets the vehicle idle for an additional 30 seconds.


A few vehicles are exempt from emissions testing, and a small group of 1982-and-newer passenger vehicles is tested with the two-speed idle test instead of with the I/M 240 test.

  • Newly-manufactured vehicles, for the first seven (7) model years, or until a change of
  • Pre -1975 collector-plated vehicles
  • Pre-1942 horseless carriage-plated vehicles
  • Vehicles with a two-stroke engine manufactured prior to 1980
  • Motorcycles
  • Farm-plated vehicles
  • Electric powered vehicles

** Hybrid electric vehicles, such as the Toyota Prius and Honda Insight, and other alternatively powered vehicles that are eight (8) years or older require an On-Board Diagnostics (OBD) test beginning Jan. 2, 2015.

For further information on exemptions, call your county motor vehicle office, or the Colorado Division of Motor Vehicles' Emissions Program Office at 303-205-5603. Also see information regarding vehicles manufactured outside the U.S. and kit cars.