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A real world aging study was performed on a distributed converter system to investigate ULEV potential and aging aspects. Two V-6 vehicles were fitted with 0.7 Liter Pd close mounted catalysts on each exhaust manifold and a traditional 2.8 Liter three-way underfloor catalyst. These vehicles were then driven in a high mileage fleet while tracking emission results every 10K miles up to 50K miles. At the end of 50K miles the system met ULEV requirements for CO and NOx but NMHC exceeded the target. Verification of aging was done by chemically cleaning the close coupled catalyst and by comparison to a dynamometer aged set. The results showed that most of the degradation in NMHC performance was due to poisoning with little effect from thermal aging. With most of the CO and NOx abatement being done in the underfloor catalyst their degradation is mainly thermal.