GMC 2006 Sierra 1500 Pickup Owner's Manual page 329

The front bumper lower air dam is held in place by a
series of push-pins located around the lower edge of
the front bumper. The push-pins are accessible from
underneath the front bumper. The following steps must
be performed on each of the push-pins to remove the
air dam:
1. Insert a tool into the
push-pin slot and pull
downward until the
push-pin snaps loose.
2. While continuing to
pull downward on the
push-pin, squeeze and
turn the expandable
end of the push-pin with
a tool until it releases
from the retainer.
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