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Reducing light-off time by using low thermal mass preconverters and increasing catalyst conversion efficiencies is one way of reducing cold start emissions and attaining mandated emission standards. Additionally, it is desirable to attain these features in a small, flexible package helping to overcome close coupling design constraints. Such a preconverter with an atypical Microlith™ metal catalyst substrate geometry and capable of withstanding high operating temperatures using a proprietary catalyst coating technique was tested with a conventional downstream ceramic main brick. This paper explains the physical characteristics and the flow dynamics of this substrate. Development and testing of prototypes were done on a bench scale apparatus and in automobiles. Bench scale catalyst performance and automotive FTP data before and after aging are presented. Automotive tests were done with and without secondary air and with calibrated Air/Fuel bias.