GMC 2003 Yukon Owner's Manual page 305

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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