Your system may have lost its mind.  Try the procedure below to find out if
you can reprogram it:

Document ID# 479317
1998 Pontiac Trans Sport
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Automatic Door Lock Feature Customization

Automatic Door Lock Operation Modes

Shift out of PARK lock enabled (modes 2, 3 and 4): Locks all doors when
shifting the transaxle out of PARK with the driver's door closed.
Ignition OFF unlock enabled (mode 3): Unlocks all doors when turning the
ignition switch from the RUN to the UNLOCK position.
Shift into PARK unlock enabled (mode 4): Unlocks all doors when shifting the
transaxle into PARK.
Changing the Automatic Door Lock Operation Mode

Perform the following procedure to change the automatic door lock operation
mode:

Turn off the IP dimmer switch.
Turn the ignition switch to the LOCK position.
Remove the BCM PRGRM fuse.
Turn the ignition switch to the ACC position.
The BCM sounds the chime a number of times equivalent to the BCM type
installed on the vehicle (1, 2, 3, or 4 chimes).
Now the BCM has entered the feature customization mode.
To change the automatic door lock operation mode, press and release the LOCK
on either door lock switch.
The BCM sounds the chime a number of times equivalent to the automatic door
lock operation mode.
A transition past mode 4 repeats the sequence starting with mode 1.

The automatic door lock operation modes are as follows:

MODE
Shift Out Of Park Lock
Ignition Off Unlock
Shift Into Park Unlock

1
Disabled
Disabled
Disabled

2
Enabled
Disabled
Disabled

3
Enabled
Enabled
Disabled

4
Enabled
Disabled
Enabled

To exit and save the current customization mode:

Turn the ignition switch to the lock position.
Reinstall the BCM PRGRM fuse.
If unable to change lock modes, refer to Feature Customization Mode
Inoperative for diagnosis and repair.