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Hi, my name is***** can also be caused by vacuum leaks or low fuel pressure or a faulty mass air flow sensor. Ideally you should have the intake system smoke tested for vacuum leakage. Small leaks can be difficult to locate without a smoke machine. A faulty brake booster could also cause the problem. Fuel pressure should be tested to be sure it is between 55-60 psi. If you are not using genuine Mercedes mass air flow senors you could be getting defective sensors.
Let me know if you have any questions.
You should be looking at the fuel trim values on the scan tool to get a better idea as to the mixture problem. More than likely they have missed an intake leak. I'd suggest smoke testing the intake. You could also block off the brake booster and see if the trim values change.