1. Bulbs: This is the most common cause. Check all three brake light bulbs (one in the center high-mounted stop lamp (CHMSL) and one in each taillight). They may be burned out, loose, or corroded. Replace them with bulbs of the correct wattage.
2. Fuses: Check the fuses in the fuse box(es) related to the brake lights. Your owner's manual will show you the fuse box diagrams and which fuses control the brake lights. A blown fuse will be visually apparent (broken filament).
3. Brake Light Switch: This switch, located near the brake pedal, activates the brake lights when the brake pedal is pressed. It's a relatively common failure point. If the switch is faulty, it won't complete the circuit, preventing the lights from turning on. This is often tested with a multimeter.
4. Wiring: Check the wiring harness from the brake light switch to the tail lights and CHMSL. Look for broken, frayed, or corroded wires. A short circuit could also be at play here.
5. Brake Pedal Position Sensor: Some vehicles use a sensor to detect brake pedal position and this can sometimes fail.
6. Tail Light Assemblies: While less common, problems within the taillight assembly itself (internal wiring or connectors) can interfere with the brake light function.
7. BCM (Body Control Module): In rare cases, a malfunctioning BCM can cause brake light issues. This is a more advanced problem and requires professional diagnostics.
Troubleshooting Steps:
1. Start with the bulbs. This is the easiest and cheapest check.
2. Check the fuses. This is also a relatively simple check.
3. Test the brake light switch. This might require a multimeter or a helper to check if power is getting to the switch and then to the lights when the brake is pressed.
4. Visually inspect the wiring. Look for obvious damage.
5. If all else fails, consult a mechanic or auto electrician. They have the tools and expertise to diagnose more complex electrical problems.
Remember to always disconnect the negative battery terminal before working on any electrical components in your vehicle.
