Transmission fault codes can be checked from the same port as any other codes, however, a basic code reader that you buy at a parts store likely won't be able to do it.

Most codes readers aren't capable of reading any codes or sensor information beyond what is required to be available by law for emissions testing. Transmission codes fall outside of that so they are typically addressed in the CAN system at the free will of auto manufacturers and the addressing is different from manufacturer to manufacturer.

It's possible you can check them with something like the Carly app or with a paid add-on to the Torque app (I haven't looked for one for Mercedes yet) through a bluetooth OBD2 CAN module. There are also probably some consumer OBD2 readers available that can be used for Mercedes specifically (that may or may not be available for a reasonable price). Otherwise you could see if a repair shop will read the codes for you (cheap or free preferably).

A long time ago, in my quest to save money by repairing my own vehicles, I made good friends with a couple guys that own a repair shop in town. When I need to I go down to their shop and use their $50k worth of computers/software to do what I want. I realize that this isn't available to the average guy on the street but...that's how I get this info for myself. It's only cost me a few drinks to earn that privilege.