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An after-treatment system consisting of a lean NOx trap, diesel particulate filter (DPF), and diesel oxidation catalyst (DOC) was applied to a light-duty commercial vehicle engine. Extensive exhaust gas testing under the transient JE05 test cycle was carried out. Lean NOx trap characteristics and issues related to transient operation were clarified and areas for system improvement were suggested. When exhaust gas temperatures were low, the NOx conversion efficiency was low, then stored NOx remained in the catalyst during rich operation, without desorption and reduction. The first half of the JE05 test cycle has relatively low exhaust gas temperatures, while the latter half has more high speed operation with rapidly increasing and higher catalyst temperature. Because of this, NOx trapped in the catalyst during the first half was desorbed and expelled during the high temperature second half, resulting in a lower NOx conversion rate for the entire test cycle.