Mercedes-Benz cars equipped with 7-Speed automatic transmission may leak transmission fluid from the O rings fittings. Leaks typically develop where the transmission lines connect to the transmission cooler/radiator. The oil leak can develop to the point that all transmission fluid is lost. The first symptoms are transmission fluid leak, Mercedes-Benz goes into limp mode, the transmission gets stuck in one gear.
As more transmission fluid is lost car no longer moves when placed in Drive or Reverse. This problem affects many models from 2005-2015 including E, C, S, ML, GL, ML, G Class and other models with 722.9 transmissions.
This is a simple DIY fix. It requires the O rings be replaced to prevent transmission fluid leak.
What is needed
First, inspect for leaks.
If any transmission fluid leaks are detected, disconnect the line.
Pull out the black cover.
You will see a C clip that needs to be removed.
Close up a picture of the clip that needs to be removed.
The transmission line can be pulled out.
O Ring can be replaced. Reinsert the transmission line and the C clip. When you disconnect the transmission line, you are going to lose some transmission oil. To minimize the leak from the disconnected transmission line either block the hose or keep it as high as possible.
Once you are done with changing the O rings, check the transmission fluid level.