Mercedes-Benz GLS Owner's Manual page 513

510 Display messages and warning/indicator lamps
Warning/indicator lamp
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Trailer tow hitch warning
lamp
Possible causes/consequences and
* The red trailer hitch warning lamp is lit.
The trailer hitch is not operational or is swivelling.
& WARNING Risk of an accident if the ball neck is not locked
The trailer may become detached.
Stop the vehicle immediately in accordance with the traffic conditions. Do not continue driving.
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Uncouple the trailer and secure it against rolling away.
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Initiate a new swivel movement and do not re-couple the trailer until the warning/indicator lamp goes out.
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Note the messages on the multifunction display.
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If the trailer hitch is swivelling:
Wait until the ball neck has reached the operational position.
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M M Solutions