A 2.5-liter light-duty diesel van certified to Euro 4 emission standards was tested in a chassis dynamometer test cell, which included a modal FTIR exhaust gas analyzer with the capability of measuring 22 separate gas species. The engine was equipped with a cooled Exhaust Gas Recirculation (EGR) system, which controls the nitrogen oxide emissions (NOx) to less than the 390 mg/km limit required by Euro 4 regulations. The vehicle was tested by dynamometer with the New European Drive Cycle (NEDC) sequence, and found to exceed the 390 mg/km NOx limit. The FTIR was applied as a diagnostic tool for the engine EGR function.The FTIR monitored N₂O, NO, NO₂, and NH₃ over the NEDC test cycles. The linear-control EGR valve failed abruptly during a subsequent test, and the relative concentration of the reduced and oxidized nitrogen species showed significant changes. The nitrous oxide increase in concentration preceded the increase in NO and NO₂ by several seconds, which suggests the EGR system failure could generate both reducing and oxidizing conditions in the LDD engine. The increase in nitrous oxide emission is interesting because it is typically an insignificant fraction of the total nitrogen mass emissions, so more nitrous oxide emitted during EGR failure is unexpected. Nitrous oxide is also of interest because it is a priority greenhouse gas that has recently been considered for regulation.After the EGR system was repaired, tests yielded NOx emissions near the 390 mg/km limit. The modal gas concentrations of the nitrogen species are shown during the EGR failure, and after the EGR repair, which resulted in low NOx and N₂O concentrations. This demonstrated the FTIR analyzer was capable of diagnosing EGR problems, and it demonstrated the value of monitoring unexpected exhaust components when an engine has failing emission control components, poisoned catalysts, or any unconventional emission problem.