This paper provides a description of automated test procedures and equipment used by the City of Chicago. Two idle testing modes are used. Emission averages for CO and HC are given for different model year range vehicles and different makes of vehicles based on tests of approximately 190,000 vehicles in 1974. The “cause of failure,” (high CO only, high HC only, or both,) is statistically analyzed and emission averages are given. A method of calculating air quality improvement from idle emission averages and other data is illustrated. The effects of seasonal variations is shown to be minimal. Results are given for a 12 month testing period in which approximately 25% of low mileage 1975 model year vehicles failed the City test. The importance of emission surveillance as a tool in improving air quality is indicated by the relatively high emission averages of “failed” vehicles in the 1970-74 model year range.