GMC 2009 Sierra - Manual (Page 64)

In addition, your vehicle has dual-stage frontal airbags.
Dual-stage airbags adjust the restraint according to
crash severity. Your vehicle has electronic frontal
sensors, which help the sensing system distinguish
between a moderate frontal impact and a more severe
frontal impact. For moderate frontal impacts, dual-stage
airbags inflate at a level less than full deployment.
For more severe frontal impacts, full deployment occurs.
Your vehicle has a seat position sensor which enables
the sensing system to monitor the position of the driver’s
seat. The seat position sensor provides information
that is used to determine if the airbags should deploy at
a reduced level or at full deployment.
Your vehicle has roof-rail airbags. See Airbag System
on page 1-54. Roof-rail airbags are intended to inflate in
moderate to severe side crashes. In addition, these
roof-rail airbags are intended to inflate during a rollover
or in a severe frontal impact. Roof-rail airbags will
inflate if the crash severity is above the system’s
designed threshold level. The threshold level can vary
with specific vehicle design.
Roof-rail airbags are not intended to inflate in rear
impacts. Both roof-rail airbags will deploy when either
side of the vehicle is struck, or if the sensing system
predicts that the vehicle is about to roll over, or in
a severe frontal impact.
In any particular crash, no one can say whether an
airbag should have inflated simply because of the
damage to a vehicle or because of what the repair costs
were. For frontal airbags, inflation is determined by
what the vehicle hits, the angle of the impact, and how
quickly the vehicle slows down. For roof-rail airbags,
deployment is determined by the location and severity of
the side impact. In a rollover event, roof-rail airbag
deployment is determined by the direction of the roll.
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