A method for monitoring performance of a passive selective catalytic reduction system includes operating the internal combustion engine in a preconditioning mode. Subsequent to the preconditioning, an air/fuel excursion is introduced into the exhaust gas feedstream and a signal output from a sensor monitoring the exhaust gas feedstream in the selective catalytic reduction system during the air/fuel excursion is monitored. An operating effectiveness is determined for the selective catalytic reduction system correlated to the signal output from the sensor monitoring the exhaust gas feedstream.