A: NOx are formed in engines when oxygen and nitrogen mix. EGR system reduces NOx by recirculating exhaust gases and cooling combustion chambers. To replace EGR valve in gasoline engines, disconnect electrical connector, remove pipe flange and bolts, and clean mounting surfaces. For diesel engines, remove cover from EGR crossover tube, loosen clamps, remove bolts, disconnect wiring, and replace gaskets. Diesel models may also have an EGR cooler.