Auto exhaust emission surveys and surveillance programs have shown 10:1 ranges in amounts of hydrocarbon and carbon monoxide emitted by individual cars. Low levels of hydrocarbon and carbon monoxide emissions can be maintained from properly tuned engines. The most practical method to reduce auto pollution would be to identify the high emitters and require proper maintenance.The purpose of this investigation was to examine existing data from auto exhaust emission surveys and surveillance programs in order to recommend a practical and efficient testing procedure for auto smog inspection. A new mathematical approach is used to discover which modes of the seven mode standard test are most discriminating for the high emitters of carbon monoxide and hydrocarbons. The idle mode is shown to be the best mode for identifying the high emitters.A 40 sec idle test is developed and applied to tune-up data. At reasonable emission level cut points, net hydrocarbon reductions over the total car population of 19% from uncontrolled cars and 13% from emission controlled cars are calculated. After maintenance the high hydrocarbon or carbon monoxide emitting uncontrolled cars showed an average hydrocarbon reduction of 49%. The emission controlled cars showed an average hydrocarbon reduction of 21%.