* Bulbs: The most common cause. Check all the brake light bulbs (both sides and the center). Even if they *seem* okay, they could be failing gradually, or have a filament broken in a way that only affects some functions. Replace them all with new ones of the correct wattage.
* Fuses: Check the fuse box (usually under the hood or in the passenger compartment) for blown fuses related to the tail lights and brake lights. Your owner's manual will show you the fuse layout and which fuses control what.
* Wiring: A common problem is corroded or damaged wiring, particularly at the connectors near the taillights. Inspect the wiring harnesses for breaks, loose connections, or corrosion. Pay close attention to the grounds. A poor ground connection can cause intermittent or complete failure.
* Brake Light Switch: This switch, located near the brake pedal, tells the system when the brakes are applied. If it's faulty, it might not be sending the correct signal to all the brake lights, or only to some of them. Testing this switch requires a multimeter to check for continuity.
* Multi-Function Switch (Often called a turn signal switch): Sometimes, a failing multi-function switch can affect brake light function. This is less common than a bad brake light switch, but possible.
* Body Control Module (BCM): In some cases, a faulty BCM can cause problems with the lighting system. This is less likely than the other issues, but a possibility if other electrical components are acting strangely. Diagnosing this requires a scan tool.
Troubleshooting Steps:
1. Check the bulbs first: This is the easiest and cheapest thing to check.
2. Check the fuses: This is also relatively simple.
3. Visually inspect the wiring: Look for any obvious damage or corrosion.
4. Test the brake light switch (with a multimeter): This requires some electrical knowledge, but you can find videos online demonstrating how to test this switch.
5. If all else fails, consult a mechanic: If you're not comfortable working with car electrical systems, a mechanic can diagnose the problem more accurately.
Remember to always disconnect the negative battery terminal before working on any car's electrical system. Safety first!
