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Motor vehicle emission testing during on-road driving is important to assess a vehicle’s exhaust emission control design, its compliance with Federal regulations and its impact on air quality. The U.S. Environmental Protection Agency (EPA) has been developing new approaches to screen the characteristics of vehicle dynamic emission control behaviors (its operating signature) while driving both on-road and on-dynamometer. The so-called “signature device” used for this testing is equipped with an O2/NOx sensor, thermocouple and GPS to record dynamic exhaust NOx concentration, air fuel ratio-controlled tailpipe lambda (λ), tailpipe temperature and vehicle speed (acceleration). In the early EPA research, signature screening was used to characterize a vehicle’s PCM control behaviors (cause/effect bijectivity), which help distinguish operation in normal control state-space and abnormal state-space.