EGR Valve
WARNING: This page does not describe the selected car, but rather 33 other vehicles, including the 2002 Saturn Vue, 2002 Pontiac Montana, 2002 Pontiac Aztek, 2002 Oldsmobile Silhouette, and 2002 Oldsmobile Bravada. However, it is still accessible from the selected car via links, so may be relevant.
EGR valve reintroduces a small amount of exhaust gas into combustion chamber, diluting air/fuel mixture and reducing combustion chamber peak temperatures, thereby reducing NOx formation.