That is a vacuum transducer which controls the EGR valve. There is another one on the right side of the car which controls the turbo wastegate actuator. They do make a sort of bleeding off noise for a minute or two after shutdown so that is normal for them, at least it is on my car.

The one on my wastegate actuator went bad about 18 months ago and threw a check engine light and the car drove very lethargically. I replaced it and all was fine for about a year and I got another CEL, same code, accompanied by the same loss of power I got the first time it failed.

This time I only replaced the small filter element which is on the end of the bleed line (on both the EGR and wastegate solenoids) and reset the CEL and never had a problem again. These transducers are not difficult to replace, they cost about $120 as I recall and the filters are just a few dollars each. They are apparently very sensitive to that little filter getting constricted so if you ever get a CEL code indicating one has failed, replace the filter first and see if that doesn't clear it up.

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz