GMC Sierra Owners Manual
The vehicle may have the delayed locking feature. When locking the doors with the power lock switch and a door is open, the doors will lock five seconds after the last door is closed. You will hear three chimes to signal that the delayed locking feature is in use.
Pressing the power lock switch twice will override the delayed locking feature and immediately lock all the doors.
This feature will not operate if the key is in the ignition.
This feature can be programmed using the Driver Information Center (DIC). See “Delay Door Lock” under Vehicle Personalization (With DIC Buttons) on page 5‑52.