In order to simulate the aging of the emission control system over the life of a vehicle, a variety of engine dynamometer cycles has been developed that accelerate the catalyst aging processes.This paper investigates the sensitivity of automotive emission control catalysts to their aging and testing environments. To show the effects of aging conditions on catalytic performance, three engine stand dynamometer aging cycles were utilized. These aged catalysts were then tested on two test vehicles: one featured multi-point fuel injection; the other, sequential electronic fuel injection.The performance results indicate the sensitivity of a catalyst to aging conditions as well as the various parameters of its operating environment. This sensitivity varied for the two catalyst formulations. The more severe aging cycle was quite successful in discriminating between catalyst formulations while the milder cycle did not greatly affect catalyst performance and hence did not clearly differentiate between formulations. The fuel control strategy of the test vehicle was a very significant factor influencing the efficiency of the catalyst formulation.