Pontiac Montana SV6 2008 Owner's Manual page 80

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 and seat-mounted side impact airbag
(if equipped), 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 and seat-mounted side impact airbag
(if equipped).
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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.