GMC 2004 Sierra - Manual (Page 317)

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.
3. Pull the push-pins and
lower dam assembly
away from the retainers
until the lower dam
is free.
4-21