Mercedes-Benz AMG GT Operator's Manual page 451

Warning/indicator lamp Possible causes/consequences and
8 * The yellow fuel reserve warning lamp lights up while you are driving.
There has been pressure loss in the fuel system. The fuel filler cap is not closed correctly or the fuel system is leaking.
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Fuel reserve warning lamp
If the fuel filler cap has already been closed correctly:
flashes
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8 * 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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Fuel reserve warning lamp
lights up
Solutions
M
Close the fuel filler cap.
Consult a qualified specialist workshop.
Refuel.
Display messages and warning/indicator lamps 449