The EGR (acronym for Exhaust Gas Recirculation) is the exhaust gas recirculation valve. Its function is to reintroduce part of the exhaust gases into the car's intake. Although it is counterintuitive, what this does is to reduce the temperature of the exhaust gases, and therefore the emission of nitrogen oxides - one of the most harmful pollutants emitted by internal combustion engines. EGR is electronically controlled in most modern cars. In older cars it is actuated by a vacuum line. However, its operation follows the same principle in both cases.