A new technique has been developed to assess the deterioration of hydrocarbon lightoff during FTP Phase 1. This method maps the dynamic real time data onto the hydrocarbon conversion/gas phase temperature plane to show the instantaneous hydrocarbon activity as a function of the exhaust temperature. Clearly, the map exhibits an apparent hysteresis bifurcation. The bifurcation is thought to be predominantly related to catalyst surface temperature. The hysteresis envelope of the map expands with aging severity. Therefore, the map can be used as a measure of catalyst deactivation.