Mercedes-Benz 2005 M-Class Operator's Manual page 258

Practical hints
What to do if ...
Problem
/
The yellow coolant warning lamp
comes on when the engine is
running.
The coolant temperature gauge is above
248°F (120°C).
256
Possible cause
The coolant is to hot.
The coolant level is too low.
If the warning lamp comes on frequently,
there is a leak in the cooling system.
The coolant level is too low.
If the coolant level is correct, the electrical
radiator fan may be broken.
Suggested solution
Stop as soon as possible and allow
the engine and coolant to cool down.
Check the coolant level and add
coolant if necessary ( page 227).
Have the cooling system checked by
an authorized Mercedes-Benz Light
Truck Center.
Stop as soon as possible and allow
the engine and coolant to cool down.
Add coolant to prevent engine from
overheating ( page 227).
If the coolant temperature is below
the red zone, immediately to the near-
est authorized Mercedes-Benz Light
Truck Center.
Avoid high engine loads (e.g. driving
uphill) and stop-and-go driving.