With ever more stringent CO2 emissions mandates, many automobile manufacturers are seeking the fuel economy benefits of diesel and lean-burn gasoline engines. At the same time the emissions standards that diesel and gasoline engines will have to meet in the next decade continue to reduce. Proposed solutions for meeting the stringent emissions standards all appear to have limitations, such as propensities to poisoning from sulfur, narrow operating temperature windows, and requirements for controls that give rapid rich excursions. Non-thermal plasma-catalyst systems have shown good performance in bench testing while being largely unaffected by these same issues. A two-stage system with a unique non-thermal plasma reactor combined with a zeolite-based catalyst has been constructed and shown to work over a wide temperature range. Exhaust from a 2.0 ℓ Opel Direct Injection (DI) diesel engine has been treated, achieving at least 60% nitrogen oxide (NOx) conversion efficiency at idle using a slipstream device. A larger device treats the entire exhaust of the Opel and achieves similar results. In both cases the exhaust is carried in heated lines before its composition is monitored simultaneously for NO and NOx with chemiluminescent NOx analyzers. Further characteristics of these systems as well as their possible application to production systems will be discussed.