This paper describes the exhaust emission performance of 127 General Motors automobiles during their first year of operation. All were 1971 models in private service. The evaluation covered the period from November 1970 to November 1971.Each car selected for the program had received an exhaust emission test before leaving its respective assembly plant. At approximately three-month intervals each car was again evaluated for emissions. It was checked for drivability and for conformance to certain emission-related specifications. Emission-related parameters were maintained at the manufacturer's specification.The diagnosis and maintenance performed kept the emission levels near their “end-of-line” averages throughout the length of the program. The causes of the emission failures were determined to define design requirements of future emission systems.