A method for indicating a non-urea reductant fault in a powertrain including an internal combustion engine and an aftertreatment system having an aftertreatment device utilizing a catalyst to convert NOx includes monitoring a NOx content exiting the aftertreatment system subsequent to a powertrain start-up event through a test span, monitoring a measure of NOx content entering the aftertreatment system subsequent to a powertrain start-up event through the test span, and determining a measure of NOx conversion accomplished in the aftertreatment device based upon the NOx content exiting the aftertreatment system and the measure of NOx content entering the aftertreatment system. The method includes, through the test span, comparing the measure of NOx conversion determined at a start of the test span to the measure of NOx conversion at a later time within the test span. A non-urea reductant fault is indicated based upon the comparing identifying a reduction in NOx conversion accomplished in the aftertreatment device between the start of the test span and the later time within the test span.