To maintain the gained low-emission behavior of Otto and diesel vehicles an effective periodical control is absolutely necessary to identify and repair high emitting vehicles. Only this method can lead to an effective reduction of a fleet's average emission level, definitely causing the actual environmental pollution. On the basis of a spot-check of about 1,000 vehicles with various types of engine measuring results of different methods will be compared and collated against the results of type approval tests (FTP 75). It will be demonstrated that the control of the engine's tune-up specifications combined with a conversion control of the catalyst converter is most suitable with regard to reliability and practicability. For diesel engines the full-load black smoke test allows the unmistakable identification of high emitting vehicles because of its correlation with the type approval test (ECE R 24).