GM Service Manual Online
For 1990-2009 cars only

If your vehicle has power door locks, it will have this feature. If you press the power door lock switch when the key is in the ignition and any door is open, all the doors will lock and the driver's door will unlock. Be sure to remove the key from the ignition when locking your vehicle.

The lockout protection can be overridden by pressing and holding the power door lock in the lock position for three seconds.