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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.