This Master Thesis deals with experimental tests performed to validate the after treatment system of a Kohler KDI 3404 diesel engine used on off-road applications, such us agricultural or earth-moving machines. In the first chapter is given an overview on the main characteristics of Diesel engines, with an analysis of pollutant substances emitted by these systems. The emission limits adopted, over the course of the years, in Europe and USA is presented in chapter 2. The experimental tests performed on the after treatment system are presented in chapters 3, 4, 5. In particular in the third chapter the fine-tuning of the DeSOx strategy acting on the SCR system of a Tier4 KDI 3404 engine is analyzed.