Systems and methods are described for performing a diagnostic on an exhaust gas aftertreatment system. A gas entry parameter into a portion of an exhaust system of an engine is determined. In response to determining that the gas entry parameter is less than the predetermined threshold, a level of NH3 in the exhaust gas is determined. In response to determining that the level of NH3 is above a threshold value, degradation of a particulate filter of the exhaust gas aftertreatment system is indicated.