Abstract: An on-board vehicle emissions testing system includes an instrument module adapted to be detachably connected to the exhaust pipe of a vehicle to provide for flow of exhaust gas therethrough. The instrument module includes a differential pressure probe which allows for determination of flow rate of the exhaust gas and a gas sampling tube for continuously feeding a sample of the exhaust gas to a gas analyzer. In addition to the module, the on-board emission testing system also includes an elastomeric boot for detachably connecting the module to the exhaust pipe of the vehicle, a gas analyzer for receiving and analyzing gases sampled within the module and a computer for calculating pollutant mass flow rates based on concentrations detected by the gas analyzer and the detected flow rate of the exhaust gas. The system may also include a particulate matter detector with a second gas sampling tube feeding same mounted within the instrument module.