* Door Switches: The most common cause. These switches are located in the door jambs and are activated when the door opens. One or more of these switches may be faulty, dirty, or misaligned. They could be broken, have corroded contacts, or simply be worn out. Test each door switch individually for continuity with a multimeter.
* Wiring Harness: The wires connecting the door switches to the interior light circuit could be damaged, broken, or corroded, especially where the wiring enters the door itself (a common wear point). Look for any obvious damage to the wiring loom.
* Body Control Module (BCM): The BCM is the "brain" that controls many electrical functions in your vehicle, including the interior lights. A faulty BCM could be preventing the signal from the door switches from activating the lights. This is less likely, but possible.
* Interior Light Switch: Even though you can manually turn the light on, there might be a problem with the switch itself, impacting the automatic activation.
* Fuse: While less likely if the manual switch works, a blown fuse specific to the door-activated lighting could be the issue. Check your owner's manual for the fuse diagram and check the relevant fuse.
* Ground Connection: A poor ground connection somewhere in the circuit could prevent the lights from working properly when triggered by the door switches.
Troubleshooting Steps:
1. Check the fuses: This is the easiest and quickest check.
2. Inspect the door switches: Visually inspect the switches in each door jamb for any obvious damage. If possible, test them with a multimeter to verify continuity when the door is opened and closed.
3. Examine the wiring: Carefully inspect the wiring harness from the door switches to the interior light assembly for any breaks, frays, or corrosion. Pay close attention to the area where the wiring enters and exits the door.
4. Test the ground: A multimeter can check for a proper ground connection at the light fixture and relevant points in the circuit.
5. Consider the BCM: If all else fails, a faulty BCM is a possibility. This requires more advanced diagnostic tools or a professional mechanic to diagnose.
Start with the simplest and most likely causes (switches and wiring) before moving to more complex components like the BCM. If you're not comfortable working with car electronics, it's best to take your vehicle to a qualified mechanic for diagnosis and repair.
