Mercedes-Benz S-Class Owner's Manual page 611

Warning/indicator lamp Possible causes/consequences and
Observe the messages on the multifunction display.
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The red trailer tow hitch warning lamp is on.
The trailer tow hitch is not operational or swivels.
& WARNING Risk of an accident if the ball neck is not locked
Trailer tow hitch warning
lamp
If the ball neck is not in the locked position, the trailer may come loose.
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Observe the messages on the multifunction display.
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If the trailer hitch swivels:
Wait until the ball coupling has reached the operational position.
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Pull over and stop the vehicle safely as soon as possible, paying attention to road and traffic conditions. Do
not continue driving under any circumstances.
Uncouple the trailer and secure it against rolling away.
Initiate a new swivelling procedure and do not re-couple the trailer until the display message disappears.
Display messages and warning/indicator lamps 609
Solutions