Various components of new technology exhaust emission control systems were disabled on five Automobile Club of Southern California fleet vehicles. Prior to and following disablement, a Federal Test Procedure and Highway Fuel Economy Test was performed on each vehicle. In addition, four short test methods were performed to determine their effectiveness in detecting the disablements. Results indicate the effects of the disablements on exhaust emissions, fuel economy, driveability and horsepower; and show the detection rate of the short test methods.Conclusions and recommendations are made regarding the effects of the disablements and the potential for preventing and/or minimizing those effects. The ability of different test methods to identify disabled emission control systems are discussed. Recommendations are made for further research with emphasis on testing of current and future emission control systems, and methods by which system failures can be identified.Computer-controlled exhaust emission control systems are currently being used by the majority of the automobile manufacturers, to meet more stringent exhaust emission standards. Because these systems have been in use for a relatively short period of time, information about them has been limited, and a need exists to collect and analyze data to determine in-use effects of this new technology. The data can be used for a variety of purposes including: predicting emissions in future years; determining the accuracy of existing emission models’ and determining system reliability (failure rates and emission deterioration rates). Other concerns relate to the potential for reduced vehicle fuel economy, and deterioration of a vehicle’s horsepower and driveability.Therefore, the Automobile Club of Southern California (ACSC) conducted a test program using its 1981 fleet vehicles in an effort to better understand the effects of system component failures. The primary objective of the program was to determine to what extent the disablement of key components of a computer-controlled emission control system affect fuel economy exhaust emissions, horsepower and driveability. The secondary objective of this test program was to establish a methods accurately and efficiently identifying vehicles with disabled components.