...is in the black box located where the battery would be in the engine compartment. Upper left corner as you look at the engine from the front of the car. It may well be the problem as I just had to replace mine. Same symptom, the check engine light stayed on. Dealership pulled the code for a faulty MAF sensor, which I replaced. As soon as I left the dealer the light came back on. Dealership pulled the code for faulty TBA. Replaced that twice and the same problem came back. Dealership said they didn't know what else to do. Took the car to an independent shop where they checked each of the other components that could cause a fault. Narrowed it down to a faulty module but couldn't say 100% that was the problem. Found a used module and problem solved.

Point of this story is that some problems on this model (mine was an E420 M119 motor) are difficult to diagnose and you may spend many $$$ before you get to the root of the problem which wasn't diagnosed by a trouble code.

Try Potomac Auto in Frederick, MD and ask for Pat. I believe their website is pgauto.com.

Good Luck!