To answer your first question, yes, the problem can be determined even if the service light is out. When the engine service light comes on, it is because a fault code is being stored in the computer. This information is held in the computer until it is scanned and cleared. You may or may not have noticed any poor performance as a result of a malfunctioning EGR valve. It would depend on the degree of failure
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Wednesday, January 26th, 2011 AT 12:44 AM