Lincoln 2009 MKS Owner's Manual page 76

Driver Controls
Cause
The surface of the radar in the
grille is dirty or obstructed in
some way
The surface of the radar in the
grille is clean but the message
remains in the display
Heavy rain or snow is interfering
with the radar signals
Swirling water, or snow or ice on
the surface of the road may
interfere with the radar signals
WARNING: Do not use ACC when towing a trailer with brake
controls. Aftermarket trailer brakes will not function properly
when ACC is activated because the brakes are electronically controlled.
Failing to do so may result in loss of vehicle control, which could result
in serious injury.
WARNING: Do not use tires sizes other than those
recommended because this can affect the normal operation of
ACC. Failing to do so may result in a loss of vehicle control, which
could result in serious injury.
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Action
Clean the grille surface in front of
the radar or remove the object
causing the obstruction
Wait a short time. It may take
several minutes for the radar to
detect that it is no longer
obstructed
Do not use ACC in these
conditions because it may not
detect, warn, or respond to
potential collisions.
Do not use ACC in these
conditions because it may not
detect, warn, or respond to
potential collisions.