Laboratory studies have indicated that closed-loop air/fuel control system characteristics have an effect on the performance of three-way catalysts (TWC). For this paper, this effect was examined on various production automobiles. Studies were conducted on three vehicles which demonstrated an observable range of air-fuel control strategies ranging from tight to wide air-fuel perturbation. To classify control system performances, data in the form of lambda (λ) traces was collected using an NGK AF-100 air-fuel ratio meter during operation of the 1975 U.S. Federal Test Procedure (FTP). Emissions performance on these three vehicles during the hot stabilized phase of the FTP cycle and for subsequent sweep test evaluations are described for two three way catalysts having different engine aging histories.