Abstract: A method for detecting contamination in diesel exhaust fluid (DEF) used in a selective catalytic reduction process is disclosed. The method may include providing a plurality of temperature sensors arranged along a fluid column within a tank containing the DEF to be tested. The method may further include detecting a temperature of the fluid at each of the plurality of temperature sensors along the fluid column, determining temperature gradients along the fluid column from the temperatures detected by the plurality of temperature sensors, and comparing the temperature gradients along the fluid column with a set of reference temperature gradients.