Well It happend to me the the car starts onlny with the fisrt gear till 50 km max .I went to mercedes service , after the Star diagnosis the repsrt of the N3 sensor reporting fals gearx problems .
Its a black plastic with 2 sensors inside the gearbox wich must be repalced.
This is a comon problem fault from the factory in S 300 diesel and benzine .

Thnx