Mercedes-Benz C-Class Saloon Owner's Manual page 511

Safety systems
Warning/indicator lamp
Possible causes/consequences and
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The yellow brake system warning lamp is lit while the engine is running.
& WARNING Risk of an accident due to a brake system malfunction
If the brake system is malfunctioning, braking characteristics may be impaired.
Brake system warning lamp
(yellow)
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Drive on carefully.
Have the brake system checked immediately at a qualified specialist workshop.
Adjust your speed and continue to drive carefully, leaving a suitable distance to the vehicle in front.
If the multifunction display shows a display message, please observe this.
Consult a qualified specialist workshop.
Display messages and warning/indicator lamps 509
M M Solutions