The pathbreaking papers by Held and Iwamoto has led to a reinvigorated search for lean NOx catalysts worldwide. Extensive effort is currently being expended to develop precious metal lean NOx catalysts, with application intended for diesel engines and low-temperature lean-burn gasoline engines. Here we concentrate on the properties of such a catalyst, referred to as LNX3. The NOx reduction selectivity of LNX3 is examined with model reducing agents. Hydrocarbons are more effective at reducing NOx than CO or H2. Next it is shown that excessively high HC/NOx ratios can degrade the performance of noble metal lean NOx catalysts. This is because the exotherm from the hydrocarbon oxidation can increase the temperature in a significant portion of the catalyst bed beyond the effective temperature window wherein the catalyst is able to reduce NOx. Depending on the concentration of NOx, the need for adequate HC/NOx ratios in order to perform the reduction will have to be balanced with this thermal consideration. Next we demonstrate the performance of LNX3 during hot-start ECE+EUDC tests on a European light-duty diesel vehicle. Although overall performance in native exhaust is inadequate, NOx conversions as high as 34% are observed when the HC/NOx ratio was in excess of 2.0. The catalyst also demonstrates good control of CO, HC, and the soluble organic fraction of the particulate on these hot-start tests. These results demonstrate the feasibility of obtaining effective “four-way” performance from such a Lean NOx catalyst in Diesel exhaust, provided the overall system is optimized to meet catalytic requirements.