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Palladium-rhodium three-way catalysts (Pd/Rh TWCs) demonstrate the most versatile capability in achieving California LEV/ULEV emission standards for various applications ranging from thermally severe high-temperature close-coupled locations to thermally limited cooler underfloor environments. Engine aged dual-brick converters consisting of front Pd catalysts followed by rear Pd/Rh TWCs met ULEV emission standards for a high-temperature close-coupled 4.3L medium-duty truck and LEV standards for a much cooler underfloor 3. IL passenger car location, as well as intermediate environments experienced by a 3.8L vehicle. The Pd + Pd/Rh TWC systems have performance advantages and are more cost effective compared to Pd-only or trimetal (Pt/Pd/Rh) systems. For thermally limited systems such as the 3. IL system, addition of air to the Pd + Pd/Rh system was necessary.