The long term durability of a heavy duty gasoline truck converter is addressed by examining thermal stresses due to radial temperature gradients under three different driving schedules. The pertinent physical properties of a catalyzed cordierite ceramic converter, with triangular cell structure, are first measured as function of temperature. These are followed by thermal mapping of mid-bed temperatures with the aid of thermocouples under various driving cycles on the truck dynamometer. Both the physical properties and the temperature distribution are then used as input parameters in the finite element thermal stress model to compute stresses in the oval converter. A comparison of thermal stresses with the high temperature strength of the mounted converter shows that the brittle fracture of a honeycomb ceramic monolith can be minimized by reducing the temperature gradient and increasing the mounting pressure via the use of a thick, high density, intumescent mat between the monolith and can.