(Updated on April 13, 2022)
Brake lights are an essential part of vehicle safety. These lights turn on when you apply the brakes, indicating to drivers behind you that you will be slowing down.
If your brake lights stay on continuously, drivers behind you will not have as much warning when you need to slow down. This increases the likelihood that you or someone behind you could be rear ended if traffic needs to stop suddenly.
How Do Brake Lights Work?
Brake lights are activated via a switch next to the brake pedal. When you first press the brake pedal, this switch will close a circuit that brings power to the brake lights.
This system is different than your normal tail lights. Tail lights are dimmer than brake lights and remain illuminated when your headlights are on. Tail lights help other drivers see your vehicle in low visibility conditions, such as nighttime or inclement weather.
If your tail lights don’t work, this is a different problem. The solution is often as simple as replacing the tail light bulb (or bulbs) that have burnt out.
Tail lights are usually fairly straightforward to replace for the DIY home mechanic, but each make and model is different. See if you can find a video on YouTube for your specific make and model to make the job easier.
Reasons Your Brake Lights Won’t Turn Off
Here are some simple things you can check that may help you determine why your brake lights won’t turn off.
1) Riding the Brake Pedal
If you prefer to brake with your left foot, you may find yourself resting your left foot over the brake pedal while you drive. It’s very easy to lightly press on the brake pedal when you do this, which will trigger your brake lights before you even feel any braking force.
Before sinking too much time or money into the problem, make sure you’re practicing good braking habits. If you rest your foot on the brake pedal while you drive, you will experience more brake pad and rotor wear.
You may also overheat the brakes if you do this over a long period of time. This could result in increased stopping distances, and even failure of the braking system altogether.
2) Broken Brake Light Switch
If the circuit for the brake light switch were to be stuck closed for some reason, your brake lights would stay illuminated all the time. This can happen due to a faulty brake light switch.
If you have a broken brake light switch, you may notice other symptoms such as not being able to start the car when the car is off, or not being able to shift out of park. On certain vehicles (such as those equipped with push button start), the vehicle needs to detect that the brake pedal is being pushed in before the vehicle will start.
On newer vehicles, a bad brake light switch may cause a number of really strange symptoms that are seemingly unrelated to the braking system. These include a tire pressure warning light, an ABS warning light, and a traction/stability control light. These may present themselves while driving.
3) Bad Ground or Wiring
This is one the most likely cause of a brake light that won’t turn off. Bad wiring or grounding issues can present all sorts of problems, depending on which area of the wiring harness is shorted or grounded improperly.
How To Fix Brake Lights That Won’t Turn Off
1) Double Check Your Work
If you’ve recently installed an aftermarket system that requires removal or replacement of the tail light assembly, it would be a good idea to go back and check your work. If you modified any of the wiring, check to make sure that it is wired correctly according to the aftermarket manufacturer’s instructions.
2) Inspect the Wiring
Inspect the wiring around your tail light assembly and under the dash near the brake light switch. You may have a wiring issue, even if the wiring and insulation appears to be in good shape.
Additionally, it’s always wise to check your fuses under the hood and in the cabin. If a fuse is blown, this could cause issues with the brake light switch operation.
You can also check the factory service manual to see which grounds are used by the braking system. Check to make sure these grounds are clean, tight, and in good working order.
3) Test Your Brake Light Switch
A brake light switch is a fairly simple device that you can probably test yourself by using a multimeter. When the switch is pressed, the circuit should be open and you should not have continuity. This turns the brake lights off. When the switch is released, you should have continuity. This turns the brake lights on.
After you have verified that you have continuity, check that the resistance being passed through the switch is correct. It is possible for a bad switch to pass a continuity test, but not be able to transfer enough voltage to actually turn on the brake lights. The correct specifications for the switch should be listed in the factory service manual.
The wiring diagram is especially helpful for testing the brake light switch connector. This will tell you which pins you need to connect to turn your brake lights on. Sometimes the service manual contains additional diagnostic tips and expected behavior of the system.
Four Pin Brake Switches
There may be different brake light switch implementations on different vehicles. For instance, you may have four pins instead of two pins (Honda, Hyundai, and Mazda are examples of manufacturers who have done this). Your best bet for correctly testing a brake light switch is to look up a wiring diagram for the vehicle to see which pins on the switch run to power, and which run to ground.
Four pin switches often have two switch circuits. These two circuits are designed to give opposite signals when the switch is working correctly (for instance, when switch 1 is on, switch 2 is off). The ECU uses this logic to test that the brake switch is working correctly. If both switches indicate they are on or both indicate they are off, the ECU knows there is a problem and throws a code. An example of a code you may see is P0504 “Brake Switch A/B Correlation”.