Mercedes-Benz E-Class Saloon Owner's Manual page 563

Warning/indicator lamp Possible causes/consequences and
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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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M M
Initiate a new swivelling procedure and do not re-couple the trailer until the display message disappears.
Display messages and warning/indicator lamps 561
Solutions