1. Interior Door Lock Actuator Switch: This is the most common cause. The switches themselves inside the door panels might have failed. They're relatively inexpensive and easy to replace, but you'll need to remove the door panel to access them. This involves removing screws, carefully prying off the panel (avoid damaging the clips), and disconnecting electrical connectors. Replacement involves reversing this process.
2. Wiring Harness Issues: The wiring connecting the switches to the central locking system could be damaged, corroded, or disconnected. Check the wiring harness between the door and the vehicle's body. Look for broken wires, frayed insulation, or corroded connectors. Repair or replace any damaged wiring. This is often found where the wiring harness flexes as the door opens and closes.
3. Door Lock Actuator Motor: While the key works, the motor that actually moves the lock mechanism inside the door might be failing. This is less likely if the key unlocks the doors, but a weak motor might not respond to the low current from the interior switch while still having enough power for the key's higher current. Replacing the actuator is more involved than replacing a switch.
4. Body Control Module (BCM) Problems: The BCM is the "brain" that controls various electrical functions, including the power locks. While less likely given the key functionality, a faulty BCM could be preventing the switches from communicating with the locking system. Diagnosing a BCM problem requires specialized tools and expertise.
5. Fuse/Relay: While less common given the key functionality, it's worth checking the fuses and relays related to the power locks in the fuse box. Consult your owner's manual to identify the correct fuses and relays.
Troubleshooting Steps:
1. Check the fuses and relays: This is the easiest and quickest check.
2. Test the interior door lock switches with a multimeter: This requires some electrical knowledge. If you're not comfortable with this, skip to the next step. You're looking to see if the switches are sending a signal when pressed.
3. Remove the door panel: Carefully inspect the wiring harness and switches for damage. Look for broken wires, loose connectors, or corrosion. Try wiggling wires gently to see if that temporarily fixes the issue (this helps identify damaged wire).
4. If wiring seems okay, replace the suspect switch(es): This is usually the most effective fix.
5. If still not working after replacing the switches and checking the wiring, consider a faulty actuator motor or even the BCM: This requires more advanced troubleshooting or professional help.
Important Notes:
* Disconnect the negative battery terminal before working on any electrical components.
* Consult a repair manual: A repair manual specific to your 1999 Pontiac Montana will provide detailed diagrams and instructions for removing door panels and accessing the locking system components. You can often find these online (Haynes or Chilton manuals).
* If you're not comfortable working on car electronics, take it to a qualified mechanic: Attempting repairs beyond your skill level could cause further damage.
Start with the simplest steps (fuses, switches, wiring) before moving on to more complex issues like the actuator or BCM. The key working suggests the problem isn't completely within the locking mechanism itself, hence the focus on wiring and switches.
