The goal of the On Board Diagnostics is to alert the driver to the presence of a malfunction of the emission control system, and to identify the location of the problem in order to assist mechanics in properly performing repairs. In addition, the On Board Diagnostics ( OBD-II) system should illuminate the malfunction indicator light ( MIL) and store the trouble code in the computer memory for all malfunction that will contribute to increased HC emissions. Therefore, the power train is controlled by the power train control module (PCM) computer to deliver the required torque to the vehicle requested by the driver and to limit the vehicle emissions to the required minimum to meet Environment Protection Agency, USA regulations. On Board Diagnostics regulations in the USA, special reference to Light and Medium duty I.C. Engine vehicles and California Motor Vehicle Pollution Control Board ( MVOCB), the following OBD-II requirements are in force, i.e. # All vehicles emission systems and components that can affect emissions must be monitored. Malfunctions must be detected before emissions exceed 1.5 times the standard specified by EPA. # Malfunction must be detected within 2 driving cycle. If a Malfunction defected a Malfunction Indicator Light illuminated. Increasing temperature inside the cylinder, HC and CO may reduce, but does not reduce the NOx. Therefore, NOx can reduce by the application of Exhaust gas Re-Circulation (EGR). NOx control system that recycles a small part of the inert exhaust gas back through the manifold to lower the combustion temperature and this system is called Exhaust Gas Re-Circulation system