Chevrolet 2006 Silverado 1500 Pickup Owner's Manual page 90

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 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
about two minutes. This will allow the system to
detect that person and then enable the passenger's
airbag.
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