GMC 2009 Sierra 1500 Crew Cab Owner's Manual page 84

Thresholds can also vary with specific vehicle design.
Frontal airbags are not intended to inflate during vehicle
rollovers, rear impacts, or in many side impacts.
If the GVWR (Gross Vehicle Weight Rating) of your
vehicle is 8,500 lb (3 855 kg) or above, your vehicle
has single stage airbags. If the GVWR is below 8,500 lb
(3 855 kg) then your vehicle has dual stage airbags. You
can find the GVWR on the certification label on the rear
edge of the driver's door. See Loading the Vehicle on
page 4-34 for more information.
In addition, the vehicle may have dual-stage frontal
airbags. Dual-stage airbags adjust the restraint according
to crash severity. The 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.
Vehicles with dual stage airbags also have seat position
sensors which enable the sensing system to monitor the
position of the driver seat (all models) and the right front
passenger seat (light-duty regular cab and light-duty
extended cab models only). The seat position sensor
provides information that is used to determine if the
airbags should deploy at a reduced level or at full
deployment.
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The vehicle may or may not have roof-rail airbags. See
Airbag System on page 1-72. 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.