What is the purpose of EGR valve on a 2002 Ford Expedition?

The EGR valve (Exhaust Gas Recirculation valve) on a 2002 Ford Expedition, like in other vehicles, serves to reduce nitrogen oxides (NOx) emissions. It does this by recirculating a portion of the exhaust gases back into the engine's intake manifold. This lowers combustion temperatures, which in turn reduces the formation of NOx. Lower combustion temperatures also slightly reduce engine power output but the environmental benefits are prioritized.