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2000 deville
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From Wikipedia
EGR testing

With OBD II standards, vehicle manufacturers were required to test the EGR valve for functionality during driving. Some manufacturers use the MAP sensor to accomplish this. In these vehicles, they have a MAF sensor for their primary load sensor. The MAP sensor is then used for rationality checks and to test the EGR valve. The way they do this is during a deceleration of the vehicle when there is low absolute pressure in the intake manifold (i.e., a high vacuum present in the intake manifold relative to the outside air). During this low absolute pressure (i.e., high vacuum) the PCM will open the EGR valve and then monitor the MAP sensor's values. If the EGR is functioning properly, the manifold absolute pressure will increase as exhaust gases enter.
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