GM Service Manual Online
For 1990-2009 cars only

A chime will sound three times to indicate a door or liftgate is open when you try to lock the doors with the remote keyless entry transmitter or the power door lock switch. The doors will not lock, and the theft-deterrent system will not arm until all the doors are closed and five seconds have passed.

The delayed locking feature can be overridden by pressing the lock button on the remote keyless entry transmitter, or the door lock switch, a second time. The doors will lock immediately and when all doors are closed the theft-deterrent system will arm after 30 seconds.

To get the horn to chirp if a door was open during the arming process, you must press the lock button on your transmitter twice after the doors are closed.