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The application of Selective Catalytic Reduction (SCR) to control nitric oxides (NOx) in diesel engines (2010, Tier 2, Bin5) introduced significant amounts of Ammonia (NH3) and Urea to the NOx exhaust gas analyzers and sampling systems. Under some test conditions, reactions in the sampling system precipitate a white powder, which can accumulate to block sample lines, rendering the exhaust emission sampling inoperable. NOx gas analyzers used for exhaust measurement are also susceptible to precipitation within the sample path and detector components. The contamination requires immediate maintenance for powder removal to restore baseline performance. The results of experiments to eliminate the powder are presented. Analysis of the powder identifies it as ammonium nitrate (NH4NO3) and ammonium sulfate ((NH4)2SO4), which is consistent with the white crystalline precipitate.