Diagnostic system for monitoring catalyst performance

A diagnostic system for monitoring catalyst performance in an exhaust system comprises a plurality of treatment devices (20,30,50,70,80) catalytically treating an exhaust gas stream, and a plurality of gas sensors (10,40,60) for monitoring the catalyst performance of the treatment devices (20,30,50,70,80) to determine when sulfur poisoning occurs. An on-board diagnostic system receives signals from the gas sensors (10,40,60), and, based upon response time differentials between sensors (10,40,60), determines whether the treatment devices (20,30,50,70,80) are experiencing sulfur poisoning.