The US EPA emission standards for 2010 on-highway and 2014 non-road diesel engines are extremely stringent, both in terms of oxides of nitrogen (NOX) and particulate matter (PM). Diesel engines typically operate lean and use at least 40-50 percent more air than what is needed for stoichiometric combustion of the fuel. As a result, significant excess oxygen (O₂) is present in diesel exhaust gas which prevents the application of the mature three-way catalyst (TWC) technology for NOX control used in gasoline engines.The objective of this work was to investigate whether or not the catalyzed DPF had a TWC-type of effect on NOX emissions and if so, why and to what extent when used on a diesel engine operating at reduced A/F ratio conditions. The approach was a two-step test plan: first to reduce the operating A/F ratio of the engine, close to stoichiometric, with an acceptable engine-out smoke increase; second, to investigate the possibility of using the catalyzed DPF (cDPF) as a PM/NOX control device at low A/F ratio operation.Low A/F ratio operation, close to stoichiometric, was achieved and tuned for low soot emissions, as indicated by the AVL filter smoke number (FSN) at five steady-state operating conditions (modes), without the aftertreatment system installed. Two aftertreatment system configurations were tested: diesel oxidation catalyst (DOC) and catalyzed DPF (DOC + cDPF) and cDPF alone. Both aftertreatment system configurations were found to have a TWC effect when the excess exhaust O₂ was reduced sufficiently and was completely consumed by the DOC and/or cDPF. This effect was observed at all five modes. The measured NOX concentration reduction ranged from 88 to 99%. The tailpipe brake specific NOX emissions were well below 0.20 g/hp-hr at the five steady-state operating conditions.