Since 1981 Colorado has operated an IM program in the metropolitan areas along the Front Range of the Rocky Mountains. Until 1995 Automobile Inspection and Repair (AIR) program consisted of approximately one thousand independently operated facilities, performing a variety of idle-tests procedures and analyzer technologies. In 1995 Colorado's AIR program was upgraded to meet USEPA requirements by implementing an enhanced IM program in the six-county Denver metropolitan area.

The enhanced IM program is a bifurcated network of centralized and independently run decentralized test-only facilities. Decentralized facilities are authorized to test 1981 and older model year vehicles employing a two-speed idle test. Centralized contractor-operated facilities test both 1981 and older vehicles using the two-speed idle test procedure, and 1982 and newer model-year vehicles using the IM240 loaded-mode transient mass emissions test. There are 15 centralized facilities operating 72 lanes and about sixty independent facilities in the enhanced program area, conducting about one million tests per year.

The primary focus of the Colorado IM program is to reduce carbon monoxide (CO) emissions. All vehicles are tested for CO and hydrocarbons emissions, while only newer vehicles subject to IM240 testing are also tested for NOx emissions levels. With phase-in cut points in place, the enhanced program is responsible for reducing CO emissions 30-34%.

As the enhanced program affects more vehicles and cut points are made increasingly more stringent, the emissions reduction benefit of the enhanced program will significantly increase.

Colorado's 1997 IM240 failure rate was 7 percent with an average per vehicle CO emission benefit per repair of 66 percent. The average cost of repairs for vehicles failing the IM240 was $189 (US). Repair costs are partially offset by an average 7 percent increase in fuel economy due to repair.

The administration of the Colorado IM program is divided among several state agencies. The Department of Public Health and Environment is responsible for the technical aspects of the program and is required to report to the Air Quality Control Commission. The Air Quality Control Commission is ensures the IM program complies with state and federal laws. The Department of Revenue is responsible for licensing facilities, inspectors and mechanics, and all enforcement aspects of the program including auditing functions.