The present paper introduces a new method of assessing catalyst performance based on inlet-outlet temperature difference measurements. These preliminary experimental results indicate that at idle speed the age and catalytic efficiency affects this temperature difference reducing outlet temperature as the catalyst ages. The reduced efficiency of the catalyst increases also the time required for the equalization of the inlet and outlet temperature. An additional phenomenon present in these experiments is the gradual appearance of nonlinear time delayed effects as catalyst age increases that are reflected in the autocorrelation function of the temperature difference signal. These effects can be used for a catalyst performance assessment using temperature probes instead of the normal exhaust gas chemical analyzers.