Urea based mobile Selective Catalytic Reduction (SCR) systems typically use a pulse width modulated injector to control the amount of reductant added to the exhaust stream. Additionally, an air assist system is provided to ensure uniform distribution of the reductant in the exhaust and to prevent injector clogging. We report on the adaptation of a commercially available pulse width modulated injector for use with a urea solution and an air assist. Flow rates and flow rate reproducibility were determined at combinations of pulse width, frequency and injector pressure drop selected to span the injector operating range. After correcting for density, deviations in flowrates were determined from the published injector calibration data when using n-heptane. These deviations were not uniform across the injector map. At the combination of low pulse width and high frequency, the deviation from the published n-heptane calibration data was the greatest. Flow rate reproducibility (for a given flow rate) was best at high frequencies and shorter pulse widths. Engine testing showed that the deviations in flow rate and injector reproducibility were transferred to the performance of the SCR system. In the absence of feed back control, high NOx conversion percentages and low ammonia slip (unoxidized reductant) when operating with a urea solution will only be achieved with precise calibration and control of the injector over the operating life of the system.