As U.S. and European regulation of automotive emissions is getting more stringent, great interest is growing around new solutions for future emission standards. Pollutant reduction can be achieved improving both engine out emission and aftertreatment system efficiency. Engine out emission can be reduced improving combustion process especially during warm-up, friction and the engine management system. In any case engine out emission reduction involves engine sophistication increasing costs, which must be accurately evaluated, especially for small displacement large mass production engine. Since, as it is well known, 80 - 90 per cent of HC and CO emissions are produced during the first 100s of NEDC cycle, great improvement could be achieved reducing the catalyst light-off time. Different configurations of exhaust gas after treatment system have been tested to improve conversion efficiency during warm-up phases. This paper reports results on precat plus underbody cat configuration behavior, comparing the effect of different aging methods (PART I). In paper part II a tool used to check threshold calibration of the most important EOBD diagnostics index during car fleet mileage accumulation, is described.