An EGR valve is a device that controls a car's emissions and is typically found between the exhaust and intake manifolds in the engine. EGR is an abbreviation for "exhaust gas recirculation," and the valve regulates how much gas from the exhaust is combined into the intake stream.
The function of the valve is to maintain temperatures in an engine's combustion chamber, ensuring that it stays cool enough and reducing how much nitrogen oxide is formed. More nitrogen oxide is formed when combustion temperatures are high. A functional EGR valve in a vehicle helps increase the efficiency of fuel burning by sending a part of the engine's exhaust into the combustion process again.