Why don’t my brake lights work

2. Checking the Brake Light Switch

If the fuse checked out okay, the next thing to check is the brake light switch.

When checking the brake light switch (or any type of car electrical circuit for that matter), it's a good idea to have on hand the repair manual for the particular vehicle model. The manual comes with the location of the brake switch, the brake light circuit diagram, and lets you identify wires and connections. This is specially important if your switch communicates with other devices like the cruise control, traction control, body control module (BCM), or transmission.

1. First, locate the switch in your car. The most common location is towards the top of the brake pedal arm. Another common location is around the brake master cylinder, on the firewall inside the engine compartment, on the driver's side.

If your vehicle has cruise control, there may be two brake light switches. If necessary, consult your vehicle repair manual.

2. Once you locate the switch, locate the electrical connector for the switch. From here, you have two ways to check the switch using a test light (this one works). Both procedures are described in the next steps.

3. Connect the test light to a good ground under the dashboard (if the switch connects to a module such as the BCM, the switch actually provides the ground connection to the module. So connect your test light to a power source, such as the battery. Consult your vehicle repair manual, if necessary).

4. Then, if you have good access to the switch connector, back probe the two terminals at the connector without unplugging the connector. Otherwise, unplug the connector and probe each terminal.
The test light should come on while probing one of the terminals but not the other one.

5. If the test light doesn't come on, the power side of the circuit (or ground, depending on your configuration) has an open or a short circuit. Use your repair manual to trace that side of the circuit and locate the fault. If the test light comes on, continue with the next steps.

6. Firmly press the brake pedal with your hand while back probing the switch connector. The test light should come on while probing both switch terminals. If the test light still comes on while probing just one of the terminals, either you need to adjust the brake light switch (if adjustable) or you need to replace it.

7. If you unplugged the switch electrical connector for this test, use a small jumper wire to connect both terminals together. This should cause the brake lights to come on. This means you should adjust the brake light switch (if adjustable) or replace it.