* Faulty Central Locking Actuator(s): The most common cause. The actuators are the small electric motors inside each door that actually move the locking mechanism. One or more may be failing, sticking, or completely dead. The passenger rear door's actuator is the prime suspect since that door won't lock at all.
* Wiring Issues: Worn, broken, or corroded wiring between the actuators and the central locking control module (usually located under the dash) can cause intermittent or complete failure. Look for breaks in the wiring loom, especially near the door hinges (where wires flex a lot). Corrosion is common in areas exposed to moisture.
* Central Locking Control Module: The module itself could be faulty. This is less likely than a bad actuator or wiring, but possible if multiple doors are exhibiting problems.
* Vacuum System (if applicable): Some older cars used a vacuum system for central locking. If your Falcon has this (less likely on a 1996 model, but check your owner's manual), leaks in the vacuum lines or a faulty vacuum pump could be the issue.
* Bad Switch: While less likely to cause only the rear passenger door to fail, a faulty switch in the driver's door could be contributing to the overall erratic behaviour.
Troubleshooting Steps:
1. Check the obvious: Ensure the doors are properly closed. A slightly ajar door can sometimes interfere with the locking mechanism.
2. Test individual actuators: If you have a multimeter, you can test the voltage at each actuator when the central locking is activated. This requires some electrical knowledge. If you're not comfortable with this, skip to the next step.
3. Inspect the wiring: Carefully examine the wiring harness running to each door, particularly the passenger rear door. Look for any visible damage, breaks, or corrosion. Pay close attention to the area near the door hinges and the rubber grommets where the wiring enters the door.
4. Try the manual lock: Can you lock and unlock the passenger rear door manually using the key or the interior door handle mechanism? If not, the problem is likely within the door itself (actuator).
5. Listen for clicking sounds: When you operate the central locking, listen carefully at each door. A weak or erratic clicking sound indicates a failing actuator.
Fixing the Problem:
* Replace faulty actuators: This often involves removing the door panel to access the actuator. Replacement actuators are readily available online or from auto parts stores. You'll need to find one specific to your car model and year.
* Repair or replace wiring: If you find broken or corroded wires, repair them with appropriate connectors or replace sections of the wiring harness. This is more challenging and might require some electrical skills.
* Replace the central locking control module: This is a more involved repair, best left to a qualified mechanic unless you have experience working with automotive electronics.
Recommendation:
If you're not comfortable working on car electrical systems, it's best to take your Falcon to a qualified mechanic or auto electrician. They have the tools and expertise to diagnose and fix the problem efficiently. Trying to diagnose and fix the problem yourself without the right knowledge and tools can potentially cause further damage.
