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This paper describes the development of an improved catalytic converter for the Volvo 850, equipped with a turbo-charged engine which could meet future legislations such as LEV and ULEV. The target has been to develop an underfloor catalytic converter with an improved light-off, OBD2 robustness and less back pressure. The new converter technology, which incorporates substrates with increased cell density and decreased wall thickness shows improved characteristics with enhanced heat and mass transfer. An investigation of metallic substrates with different cell densities shows shortened light-off times with higher cell density. An oxygen storage investigation shows also that increased cell density is a very effective way to reduce the lambda perturbations after the catalyst, giving a stable rear oxygen sensor signal. In the present case this was desirable for the dual sensor system used together with a small catalyst volume in the first position of a cascade of several substrates.