GM Service Manual Online
For 1990-2009 cars only

This feature will delay the actual locking of the doors and arming of the theft-deterrent system for five seconds when the power door lock switch or remote keyless entry transmitter is used to lock the vehicle.

If any door is open when locking the vehicle, three chimes will sound signaling that the delayed locking feature is active. Five seconds after the last door is closed, all of the doors will lock. To cancel the delay and lock the doors immediately, press the lock button on the remote keyless entry transmitter or the power door lock switch a second time. The theft deterrent system will arm after 30 seconds.