A previously developed three-dimensional, transient numerical model was extended to Pt/Rh catalyzed oxidations. The numerical model has been applied to a metal monolith converter for simulations of the light-off conversion performances and thermal response during a thermal cycling test. In this study, we consider an oval converter with a metal monolith substrate which has fully orthotropic thermal properties. The analysis accounts for transient inlet exhaust gas conditions and non-uniform inlet flow distributions. The simulation covers a wide range of exhaust gas stoichiometrics. The predicted light-off conversion efficiencies are very close to the test results. The simulated temperatures compare favorably with the measured data throughout the converter. Temperature contours are also illustrated and the temperature gradients can be as high as 500°C/cm at the monolith edge during thermal cycling.