A comprehensive vehicle emission testing program has determined that the California vehicle inspection and maintenance (I/M) program (called “Smog Check”) is reducing emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen from vehicles subject to the program by 12%, 10%, and 4%, respectively.Although substantial, the emission reductions of HC and CO are falling short of the 25% reduction goal set by the Environmental Protection Agency. Reasons for the shortfall include limitations in the ability of idle emission standards to detect defective vehicles; poor quality of visual and functional inspections; and ineffective repair of many failed vehicles. The overriding problem with the program can be summarized as the failure of participating mechanics to identify and correct emissions defects, especially in 1980 and later model vehicles.There are two types of mechanic performance problems which need to be addressed. First, mechanic training and qualifications are clearly inadequate for the effective inspection and repair of computer controlled vehicles. Second, many mechanics deliberately ignore defects that they are capable of detecting and correcting.Through changes in testing procedures, mechanic qualification requirements, and enforcement practices, existing deficiencies can be corrected and the emission reductions achievable with I/M can exceed 25% for hydrocarbons and carbon monoxide.