GM Service Manual Online
For 1990-2009 cars only

The lockout protection feature makes it more difficult to lock the key in the vehicle. If the driver's door is open while the key is in the ignition, the door cannot be locked with the power door lock switch.

This feature cannot guarantee that you will never be locked out of the vehicle. If the key is not left in the ignition, or, if the manual door lock is used, the key could still be locked inside the vehicle. Always remember to take the key with you.