Nitrogen oxides NOx are those compounds formed in the combustion chamber with oxygen and nitrogen; in the incoming air normally above 2500 F temperatures When emitted from the car tail pipe the NOx combines with the reactive organic compounds, hydrocarbons and sun light to cause formation of ozone and photochemical smog. The EGR system work to reduce NOx emission; exhaust gases from the exhaust manifold are channeled by the EGR valve and then back to the intake manifold to mix with the air/fuel mixture thereby cooling the combustion chambers. This system comprises of the PCM, EGR valve, as well as the sensors that assist the PCM to decide the right time at which to open the EGR valve. When initialised, the EGR valve, has a position feedback circuit which accurately positions the pintle valve to enhance performance of the EGR system. Too much EGR flow might cause the engine to stall or has rough idle, as little flow might cause high combustion temperature, spark knock or overheating. In the case of EGR valve replacement unplug the electrical connector, then remove the bolts and extract the valve and its old gasket, before cleaning the contact surface and replacing the gasket with a new one then fixing the valve. For EGR pipe replacement, remove the nuts that connect the pipe to the intake and exhaust manifolds, remove the pipe, clean it well, apply anti seize on the threads and offer back the pipe, make sure that all nuts are right tight.