Abstract: A system is provided for monitoring exhaust gas concentrations in an exhaust stream from an engine. The system may include the exhaust system configured to carry an exhaust stream generated by the engine. The system may further include at least one sensor in communication with the exhaust stream and configured to generate an output indicative of a concentration of NOx and a concentration of at least one sulfur-containing compound in the exhaust stream. The system may further include a controller configured to selectively produce one of a rich condition and a lean condition in the exhaust stream and to determine, during the rich condition, the concentration of the at least one sulfur-containing compound in the exhaust stream based on the output from the at least one sensor and to determine, during the lean condition, the concentration of NOx in the exhaust stream based on the output from the at least one sensor.