Why The Third Brake Light Stops Working

When you have a third brake light that stopped working, there are a few possible problems to investigate. In this post, we’ll go over the most common causes of third brake light failure. We’ll also discuss what you can do to resolve the issue.

1. Blown Fuse

Blown fuse

If all three of your brake lights stopped working, a blown brake light fuse may be the problem. In that case, you don't need to replace the third brake light. To determine if the fuse is the issue, you need to check it. To do this:

  1. Find the fuse box.
    1. The owner's manual should tell you where it is
  2. Find the fuse for the brake lights.
    1. Refer to the owner's manual if you're not sure which fuse it is
  3. Remove the fuse
  4. Examine the fuse in good lighting. If the fuse wire is incomplete, or the inside of the fuse is sooty, the fuse is blown.

2. Short Circuit

wire short

Sometimes the problem goes deeper than a blown fuse. The fuse may be fine, but there may be a short circuit in the electrical system. If you replace the fuse and it immediately blows again, there is a short in the brake light circuit. Usually this means that the car body or frame has worn through the insulation on the brake light wiring harness. When the bare wire make contact with the body or frame, the fuse blows. These can be challenging to find, but they are easy to fix. If you can't find the short, it may be best to bring your vehicle to a shop and have the professionals take care of it.

3. Burned Out Bulb

Burned out bulb

Brake light bulbs don't last forever. The longer you keep your car, the more likely you will deal with a burned out brake light bulb. The good news is this is a very easy fix. All you need to do is replace the bulb.

To determine if the bulb is the reason your third brake light isn't working:

  1. Access the third brake light bulb.
    1. If you're not sure how to access it, consult the owner's manual.
  2. Remove the bulb and inspect it.
    1. Is the bulb housing cracked? If so, the bulb is no longer good.
    2. Is the filament blackened and/or broken? If so, the bulb is no longer good.

If you've determined that the bulb has gone bad, put in a new bulb. That should take care of the problem. Chances are the other brake light bulbs will burn out soon. To make it easier for your future self, replace all three bulbs at once.

4. Malfunctioning Brake Light Switch

brake light switch

Image Credit: kre57

The brake light switch is a basic two-wire switch that controls the brake light circuit in the brake system. You'll find the switch near the brake pedal. If you're not sure how to find it, consult the service manual. To test the brake light switch:

  1. Grab an automotive test light and then attach one lead to a ground.
    1. If you don't have one you can get one at an auto parts store.
  2. Turn the ignition to the on position, but don't start the car.
  3. With no pressure on the brake pedal, touch the test light lead to each of the two wires on the switch. Only one wire should cause the test light to illuminate.
  4. Put the test lead on the wire that did not cause the tester to illuminate.
  5. Push on the brake pedal. If the switch is functioning, the light should now illuminate. If it did not, the switch is bad.

If the four tests above do not find the cause, then the problem is likely loose wiring within the brake light assembly. The wiring can come loose due to vibrations over time. If that's the case, you will need to replace the assembly. We carry genuine OEM third brake light assemblies for a variety of GM models. Find yours here!