Assuming the wiring harness has been replaced, try this before you get into a lot of diagnostics:
In the engine compartment, check every vacuum line you can get your hands on for hardening/cracking due to aging. These are the small, approximately 1/8" diam., white, maybe black, plastic lines you'll see throughout the engine compartment. Make sure they are all connected to something with a snug fit. If any duck behind the engine, follow with your fingers and make sure they attach somewhere. Very gently, squeeze and slightly bend every one with your fingers. They should be stiff but flexible. You'll feel it if one is hard and starts to crack. Those are the ones to replace. The two lines under the cover on top of the engine are especially vulnerable. Pull the cover and check them. Make sure the mating rubber connectors are soft with no cracks and the lines go into them securely. Lines and connectors are inexpensive and available at local MB dealer. You may be able to turn off the CEL light by disconnecting the battery at the negative post for a couple of minutes but if the problem is still there it will come back on. Have the radio code ready.
If the wiring harness has not been replaced or your aren't sure then search this forum for info on how to determine if your harness is bad. You can still look for bad vacuum lines but poking around near any wiring could make things worse.
Good Luck, Dan

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2012 E350 Bluetech