CITROËN DS 4 Handbook page 273

Take the towball in both hands; insert the end
of the towball into the carrier by lifting it, then,
to ensure that it has correctly locked in place,
pull firmly down on the ball end of the towball.
The locking wheel automatically turns a quarter
of a turn anti-clockwise, making a perceptible
click; take care to keep your hands clear.
Check that the towball has correctly locked in
place (position A).
Turn the key clockwise to lock the towball fixing
mechanism.
Always remove the key and keep it in a safe
place.
Practical information
If the key cannot be turned or removed,
this means that the towball is not fitted
correctly; start the procedure again.
If one of the locking conditions is not
met, start the procedure again.
In all cases, if the marking remains
red, do not use the towbar and contact
a CITROËN dealer or a qualified
workshop.
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