The effect of inlet flow distribution on the performance characteristics of a catalytic converter is analyzed for two common inlet manifold designs. Three-dimensional steady-state computational fluid dynamics simulations are performed under isothermal and nonisothermal conditions for the entire converter comprising an inlet manifold, central monolith, and outlet section. The porous medium approximation is used for the catalytic monolith. The reaction mechanism for a diesel oxidation catalyst with five global reactions is employed. A parameter called the “flow distribution index” is defined in order to quantify the nonuniformity of the velocity profile within the monolith and hence analyze the effect of operating conditions for the two geometries. Under isothermal conditions, catalytic converters with straight and U-bend inlet manifolds behave similarly and closely match single channel predictions. However, under nonisothermal conditions, there is a considerable difference in performance between the two geometries because of temperature variations. The presence of heat effects in the nonisothermal case marginally improves the flow uniformity within the monolith. However, in the presence of heat effects, reactant conversion in the center of the monolith varies from the periphery and the single channel model slightly overestimates the conversion for both geometries. © 2021 American Chemical Society.