To comply with the upcoming emission regulations for non-road applications, especially the TIER 4 Final emission legislation, a significant reduction in particulate matters and nitrogen oxide emissions is necessary. An exhaust gas after-treatment system with a good performance helps to meet these requirements.The following paper focuses on the possibilities to reduce the nitrogen oxide emissions in exhaust gas after-treatment technology using selective catalytic reduction with AUS32. Using this technology and targeting a nitrogen oxide emission reduction of 90% the exhaust gas after-treatment system enables engine-out emissions of about 3 - 4 g/kWh nitrogen oxide. Considering an increase of only 5% reduction efficiency to 95%, a duplication of engine-out emissions could be acceptable for still meeting TIER 4 Final emission legislation. This would enable the engine manufacturer to avoid internal engine measures, such as exhaust gas recirculation, and therefore provide the opportunity to save on space, cooling, fuel and costs.To achieve maximum reduction efficiency of nitrogen oxides it is important to understand the influential parameters and their interaction within the complex selective catalytic reduction system. An analysis to determine the influence of each parameter and its interaction on the overall efficiency within the TIER 4 Final emission tests for off-highway applications (the steady state C1 test and the non-road transient cycle - NRTC) has been set up on a theoretical basis. To determine the path to high reduction rates, the results of the theoretical analysis have been simulated with computational fluid dynamics (CFD) analysis and tested on an engine test bench.The paper describes this program and analyses the most influential parameters, how to achieve maximum nitrogen oxide conversion rates, and the limits of this technology within the scope of these investigations.