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Investigations of on-road emissions performance of vehicles have been made using various methods and instrumentation, some of which are very complex and costly. For the particular case of NOx emissions on Diesel road vehicles equipped with SCR catalysts (Selective Catalytic Reduction), many of these vehicles are equipped with NOx sensor(s) for the purpose of OBD (On-Board Diagnostics), and the ECU (Engine Control Unit) makes this data available via the diagnostic connector under the SAEJ1979 protocol for light duty vehicles. Data for mass air flow and fuel flow are also available per J1979, so the ongoing NOx mass flow can be estimated when the NOx sensors are active with no additional instrumentation. Heavy duty pickup trucks with SCR systems from 3 major US manufacturers, each certified to the optional chassis certification of 0.2 g/mi NOx on the FTP75, were obtained to be evaluated for SCR system behavior under normal driving conditions.