GMC 2008 Envoy Denali Owner's Manual page 68

The passenger sensing system is designed to enable
(may inflate) the right front passenger's frontal
airbag anytime the system senses that a person of adult
size is sitting properly in the right front passenger's
seat. When the passenger sensing system has allowed
the airbag to be enabled, the on indicator will light
and stay lit to remind you that the airbag is active.
For some children who have outgrown child restraints
and for very small adults, the passenger sensing system
may or may not turn off the right front passenger's frontal
airbag, depending upon the person's seating posture
and body build. Everyone in your vehicle who has
outgrown child restraints should wear a safety belt
properly — whether or not there is an airbag for that
person.
If a person of adult-size is sitting in the right front
passenger's seat, but the off indicator is lit, it could be
because that person is not sitting properly in the seat.
If this happens, turn the vehicle off, remove any additional
material from the seat, such as blankets, cushions, seat
covers, seat heaters or seat massagers and ask the
person to place the seatback in the fully upright position,
then sit upright in the seat, centered on the seat cushion,
with the person's legs comfortably extended. Restart the
vehicle and have the person remain in this position for
two to three minutes. This will allow the system to detect
that person and then enable the right front passenger's
frontal airbag.
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Safety belts help keep the passenger in position on
the seat during vehicle maneuvers and braking, which
helps the passenger sensing system maintain the
passenger airbag status. See "Safety Belts" and "Child
Restraints" in the Index for additional information
about the importance of proper restraint use.
If you ever pull the shoulder portion of the belt out all the
way, you will engage the child restraint locking feature.
This may unintentionally cause the passenger sensing
system to turn the airbag(s) off for some adult size
occupants. If this happens, just let the belt go back
all the way and start again.