A series of experimental programs was conducted to provide an empirical data base for a computer model of the economic effectiveness of mandatory inspection and maintenance of light-duty vehicles. The experimental programs provided four basic data sets: 1. The frequency and extent of engine maladjustments and component failures of in-use vehicles. 2. The change in exhaust emission levels that results from changes in engine adjustments and component condition. 3. The deterioration of engine adjustments and components and the associated deterioration in exhaust emissions with the accumulation of time and mileage. 4. The ability of automotive service organizations to detect and repair engine maladjustments and failures. The data that resulted from these experiments were effectively employed in the computer model.