Most current engines employ exhaust gas recirculation, or EGR, to reduce combustion temperature in order to mitigate the formation of oxides of nitrogen, a nasty pollutant. A small quantity of exhaust (inert) gas is combined with the incoming combustion air to take up space that would have been air and fuel. This is usually done via an electric solenoid or vacuum servo and solenoid (older) that receives signals from the powertrain control module. Various means of confirming EGR function are used so that faults can be readily identified and corrected. Occasional issues may occur due to the EGR devices failing, or perhaps clogging of the EGR passages in the cylinder heads.