Mercedes-Benz R-Class Owner's Manual page 300

Problem
The engine cannot be started
using the key.
* optional
Possible cause/consequence
The on-board voltage is too low.
Suggested solutions
Switch off non-essential consumers, e.g. seat heating* or interior
X
lighting, and try to start the engine again.
If this does not work:
Check the starter battery and charge it if necessary
X
( page 323).
Y
or
Jump-start the vehicle ( page 329).
X
Y
or
Consult a qualified specialist workshop.
X
Practical advice
Troubleshooting
Z
297