A total of 600 (1968-1974) randomly selected vehicles were screened by engine parameter, safety, and idle tests. Those cars requiring major engine repairs or safety defects were rejected. Five hundred and seventy-six (576) were selected for testing and incorporated into four test groups. Each group contains 144 vehicles and are designated as the Baseline Control Group (Group I), the Inspection Group (Group II), a Manufacturer's Specification Group (Group III), and a Mandatory Maintenance and Parameter Inspection Group (Group IV).All vehicles entering the test program received a baseline 1972 Federal CVS emission test, followed by a hot-idle test (low and 2,500 rpm). Adjustment or repair of vehicles was accomplished by Olson Laboratories for Group II and Group III vehicles. Class A garages performed maintenance on Group IV vehicles. Following the necessary maintenance, the Federal and idle tests were repeated for Group II, Group III, and Group IV vehicles. All vehicles in these groups were retested at 1- 3- 6- 9- and 12-month intervals.