A system includes a controller configured to compare a nitrogen oxides (NOX) concentration within treated exhaust gases from a combustion engine after flowing through a first catalyst assembly and a second catalyst assembly relative to a NOX threshold value, to determine a change in O2 concentration within the treated exhaust gases between the first and second catalyst assemblies upstream of a location of oxidant injection into the treated exhaust gases, and to adjust an air-fuel ratio of the combustion engine based on the change in O2 concentration in the treated exhaust gases if the NOX concentration is greater than the NOX threshold value.