GM Service Manual Online
For 1990-2009 cars only

Lockout protection is a standard feature intended to provide enhanced security and convenience. This feature prevents a driver who has left the keys in the ignition from locking the doors or using the power door locks while any door is open.

To override this feature, when the key is in the ignition and a door is open, hold the power door lock switch for three seconds.

This feature cannot guarantee that you will never be locked out of your vehicle. If you don't leave the keys in the ignition, or if you use the manual door lock, or if you use the remote keyless entry transmitter (if equipped), you could still lock your keys inside your vehicle. Always remember to take your keys with you.