Comparison of before-repair and after-repair test results for approximately 800 1981 and later model cars and light trucks recruited from customer service shows that the primary cause of excessive emissions depends on fuel metering technology (i.e., carburetor versus fuel-injection). With carbureted vehicles, mechanical component failure is the largest contributor to excessive emissions. Specifically, the need for adjustment or other repair of the carburetor is the single greatest cause of excessive emissions for carburetor-equipped vehicles. Ignition system maintenance and oxygen sensor replacement are the next most significant items.Electrical component failure is the largest source of excessive emissions for fuel-injected vehicles. Oxygen sensor failure is the single greatest source of excessive emissions and ignition system problems are second largest emissions source. Tampering with emission control systems, a significant problem with older, carbureted vehicles, appears to be a relatively minor problem with fuel-injected vehicles. Component failures occurring during the 5 year/50,000 mile warranty period contribute relatively little to excessive emissions. At higher mileages, the contribution of failed emission control computers and catalysts to excess emissions is more significant.Analysis of available data indicates that the repair of all defects identified through diagnostic inspections would reduce fleet-average exhaust emissions of fuel#x002d;injected cars and light trucks by 34-47% for hydrocarbons (HC), 46-52% for carbon monoxide (CO), and 16-21% for nitrogen oxides (NOx). For carbureted cars and light trucks, the repair of all defects would reduce fleet-average emissions by 66-76% for HC, 77-87% for CO, and 12-14% for NOx.