Mercedes-Benz S-Class 2013 Owner's Manual page 201

Driving
198
If indicator lamp ; is off, the ECO start/stop
function has been deactivated manually or as
the result of a malfunction. The engine will
then not be switched off automatically when
the vehicle stops.
If drive program S or M has been selected,
the ECO start/stop function is always deac-
tivated.
If you switch on the ECO start/stop function
while drive program S is selected, the drive
program automatically switches to C.