Mercedes-Benz 2003 M-Class Operator's Manual page 230

Practical hints
What to do if ...
Problem
/ The yellow coolant warning lamp
lights up when the engine is run-
ning.
The coolant temperature gauge is above
248°F (120°C).
230
Possible cause
The coolant is to hot.
The coolant level is too low.
If the warning lamp lights up 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 206).
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 206).
If the coolant temperature is below
the red zone, drive on to the nearest
authorized Mercedes-Benz Light
Truck Center.
Avoid high engine loads (e.g. driving
uphill) and stop-and-go driving.