Mercedes-Benz G Series Operator's Manual page 455

Warning/indicator lamp Possible causes/consequences and
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If the fuel filler cap has already been closed correctly:
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*The yellow fuel reserve warning lamp lights up while the engine is running:
The fuel supply has dropped into the reserve range.
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M M
Close the fuel filler cap.
Consult a qualified specialist workshop.
Refuel.
Display messages and warning/indicator lamps 453
Solutions