The transmission drive selector module may fail resulting in the loss of gear selection capability. This condition can leave the vehicle stranded, and will require replacement of the drive selector module.Google+
Car Problem Reports
Mercedes-Benz S550 4MATIC The Transmission Drive Selector Module May Fail
Mercedes-Benz S550 4MATIC Problem
Average Mileage: 70,239 mi (11,000 mi - 147,000 mi)
Visitor, 2007 Mercedes-Benz S550 4MATIC, 64,000 mi
Transmission stuck in one gear.(safe mode)
I've had this problem too on my 2002 C240 147K miles, then Replaced the Tranny fluid and can drive for another 300 miles then problem happened again. Already checked for no leaks of fluid.
Visitor, 2009 Mercedes-Benz S550 4MATIC, 11,000 mi
Transmission switched to low gear and stayed there - so you can only travel at about 15-20 mph. Mercedes dealership says that the electronics force it into a "safe mode" when this happens so one can not damage the transmission. Despite it being covered fully by warranty, it makes me uneasy in a car with 11k miles!!!