A diffuser plate for a vehicle exhaust system comprises a body having a primary opening and a plurality of secondary openings circumferentially spaced apart from each other about the primary opening. In one example, the diffuser plate is positioned between an exhaust manifold and a catalytic converter. Exhaust gas from the exhaust manifold flows through the primary and secondary openings in the diffuser plate to provide a thorough mixed flow that is evenly distributed across an inlet to a catalyst substrate, and also enable an oxygen sensor to accurately measure engine cylinder exhaust gas chemistry.