GM Service Manual Online
For 1990-2009 cars only

All of the doors will automatically lock when all doors are closed, the engine is running and the shift lever is shifted out of PARK (P). All of the doors will automatically unlock when the shift lever is shifted into PARK (P).

With the automatic door lock feature, you can still lock or unlock the doors at any time, either manually, with the power door lock switches or by pressing LOCK on the remote keyless entry transmitter. See Remote Keyless Entry (RKE) System Operation for more information.

If your vehicle is equipped with the Driver Information Center (DIC), see DIC Vehicle Personalization to program the automatic door lock feature.