Automobiles and other mobile sources are a major contributor to precursors of ozone and to carbon monoxide in urban area air pollution inventories. In a typical urban area, over fifty (50) percent of the emissions of hydrocarbons (HC) and of oxides of nitrogen (NOx), which react in the presence of sunlight to form ozone, are derived from vehicular sources. Over ninety (90) percent of the carbon monoxide emissions in these areas are also attributable to mobile sources. This problem is accentuated in the Front Range Region of Colorado, due to the higher altitude. The Colorado Automobile Inspection and Readjustment (AIR) Program was designed and implemented in an effort to reduce the contributions to ambient air pollution from mobile sources. The intent of this study is to evaluate whether there is a difference in the effectiveness of the Colorado AIR Program on the major technological groups of emissions control configurations.