Exhaust gas recirculation (EGR) systems in late-model vehicles serve two key purposes, lowering oxides of nitrogen (NOx) emissions and preventing significant engine knock or detonation. Again, by injecting into the intake manifold a low proportion of the exhaust, combustion temperatures are reduced since exhaust contains gases which do not burn. These combustion temperatures are higher than 2500 F and result in high NOx formation; they, in conjunction with HC and sunlight, cause photochemical smog. EGR systems are generally claimed to run with minimal problems and the failures are reported to occur when the EGR valve is closed or fully open. There may not be visible sign of a closed valve but it may cause engine detonation or overheating whenever it is under load and on the other hand, an open valve may cause rough engine operation and stalling. In component replacement, the procedure include lifting of the vehicle and removing of some connecting part such as EGR pipe and the valve in addition, the identities of all old gasket and deposits before fitting. One has to be extremely careful whenever the EGR pipe is removed from the valve, and always ensure that the pipe is changed together with the threads with any sign of deterioration. After reassembly there is need to make sure that the PCM relearns for its idle as well as for the fuel trim strategy.