* Faulty Light Switch: This is a common culprit. The switch itself might be malfunctioning, stuck in the "on" position, or have internal electrical shorts.
* Body Control Module (BCM) Issues: The BCM is a computer that controls many electrical functions, including lights. A faulty BCM, a short circuit within it, or a software glitch could keep the lights on.
* Wiring Problems: Damaged, frayed, or corroded wiring anywhere in the lighting circuit can cause a short circuit, constantly supplying power to the lights. This includes wiring harnesses near doors, under the dash, and in the engine bay.
* Door Jamb Switch Malfunction: These switches tell the car when a door is open or closed, and trigger the dome light accordingly. If one is stuck closed or malfunctioning, it might keep the interior lights on. This is especially likely if only the interior lights remain on.
* Aftermarket Accessories: Incorrectly installed or malfunctioning aftermarket accessories (stereo systems, alarms, etc.) could draw power and keep lights on.
* Short in the Lighting Circuit: A short-to-ground anywhere in the headlight, taillight, or interior light circuits will cause those lights to stay on.
* Battery Drain: While not the cause itself, a constantly-drained battery due to lights staying on is a symptom, not a cause. The battery could be weak as well, exacerbating the issue by not showing voltage drop readily.
Troubleshooting Steps:
1. Check the obvious: Ensure all doors and the tailgate are completely closed and the light switch is definitely in the "off" position.
2. Check the fuses: Inspect the fuses related to the lighting circuits in the fuse box (usually under the hood and inside the cabin). Look for blown or corroded fuses. Replace any that are blown. Note however, that a blown fuse might *result* from a short circuit, not the source of the problem.
3. Test the light switch: Try jiggling the light switch gently; if the lights flicker or turn off, you likely have a faulty switch.
4. Inspect the wiring: Carefully examine the wiring harnesses, particularly around the doors, for any visible damage, fraying, or corrosion.
5. Check the door jamb switches: See if the dome light reacts appropriately when opening and closing each door. If a switch is faulty, it's often possible to clean or adjust it.
6. Have it professionally diagnosed: If you can't pinpoint the problem yourself, take the Expedition to a qualified mechanic or auto electrician. They have specialized tools to diagnose electrical issues.
Finding the cause might require systematic testing. Start with the simplest steps and move to more complex ones as needed. Remember safety first – always disconnect the battery's negative terminal before working with car wiring to prevent shorts or shocks.
