An EGR, or Exhaust Gas Recirculation valve is a means of reducing NOx (Nitrogen Oxide) emissions from your engine. The EGR valve, as its name suggests, re-circulates exhaust gases to act as a coolant in the combustion chambers. Over time EGR valves become sticky & will eventually seize open or closed. If seized open the engine may idle poorly or even stall. If seized closed the engine will operate at increased temperatures under load & may cause "engine knock". This is especially bad for turbocharged vehicles, as the increased exhaust gas temperature can lead to premature failure of the turbocharger also.