GMC Sierra Owners Manual
The security light should come on briefly as the engine is started. If the system is working normally, the indicator light turns off. If it does not come on, have the vehicle serviced by your dealer.
If the light stays on and the engine does not start, there could be a problem with the theft-deterrent system.
This light is also used to indicate the status of the anti-theft alarm system when the ignition is turned off. The light will flash rapidly if the alarm system is arming and one or more of the monitored entry points is not closed. The light will stay on if the alarm is arming and all entry points are closed.
For information regarding this light and the vehicle's security system, see Anti-theft Alarm System on page 2‑11.