GM Service Manual Online
For 1990-2009 cars only

To protect you from locking your key in the vehicle, this feature stops the power door locks from locking while the key is in the ignition and a door is open.

If a power lock switch is pressed while a door is open and the key is in the ignition, all of the doors will lock and then the driver's door will unlock. A chime will sound continuously until all doors are closed.