There is no denying the importance of the brake light system. Because of this, it is important to understand how this lighting works, how to confirm its operation and its modes of failure will be a benefit for your safe driving.
Once in a while, you will experience a malfunction in your brake light system, in which your brake lights will not turn off. Keep reading to discover the common causes of this failure and how to fix it and do it yourself.
How Brake Lights Work?
Brake (or stop) lights are part of the rear light assembly of cars, trucks, and SUVs. Brake lights started being added to cars around 1905.
At the time, drivers were signaling to the cars behind them that they were about to stop or turn by hand. Hand signaling is common today for bicyclists and some motorcyclists and scarce for cars.
By 1928, 11 states in the US had their own requirement, although all builders were adding brake light (or lights) to the rear of their cars. This new law was needed because of increasing nighttime automobile traffic and concomitant rear end collisions. Several more years passed before brake lights became the law for the entirety of the US.
In 1974, the high, or third, brake light was added as a requirement for all cars. Because this was higher on the car than the other rear brake lights, thereby giving a more visible warning to the car behind, it additionally reduced risk from rear end collisions.
If you were to guess, pressing the brake pedal causes your rear brake lights to work. This can be done even if the engine is not running, and the ignition is off, so the question is: What makes this happen?
In most cars, there is a switch (the brake light switch) located beneath the dashboard of the driver’s side.
Our switch works much like a push-button, where its button (or plunger) touches a section of the brake pedal lever. When you push the brake, the lever moves forward, which allows the plunger to extend and activate the switch contacts, which sends a signal to the brake light system on the vehicle, and the lights come on. Simple enough, right?
Older cars made in the USA, before 2006, used this switch to send electrical power directly to the brake lights. The wire (or wires) that powered the brake lights ran straight to this switch, exactly like the wall switch and the ceiling light in your kitchen. Simple. Not so today.
Most of today’s vehicles use a computerized system called the Controller Area Network (CAN) Bus system to control many internal systems, including exterior car lighting.
This system reduces the amount of wire harnesses in vehicle construction and allows for better control and fault checking for systems that use electronic controllers such as the ECU (Engine Control Unit).
The CAN Bus system controls the brake lights also. You push the brake pedal and the CAN system sends an encoded signal to a control module. The control module powers the brake lights and they come on. A nice benefit to all this technology is a failed brake light bulb can be detected.
When a bulb fails, your instrument panel will display a warning light or a graphic display. Not only are you informed that a light is not working, but you may also see a graphic display showing the location of the failed bulb.
Most vehicles will register a fault for this failure and set a Diagnostic Trouble Code (DTC). A DTC can be retrieved with an OBD2 code reader and will report the failure caused by the electrical fault.
Reasons Why Your Brake Lights Won’t Turn Off While Driving
This can happen while you are driving, and the most problematic part is that you typically won’t know they are on. Here are some causes of this common failure:
#1. Faulty Brake Light Switch.
The brake light switch is located next to the brake pedal linkage and can fail in the open position. The switch may appear to move properly when you depress the accelerator, however, it does not break the electrical connection to turn off the brake lights.
This is an unusual failure because manufacturers design this specific switch to be very reliable. But it can fail and cause your brake lights to be energized and stay ON.
The switch is relatively simple to replace, assuming you’re able to contort your body to gain access. If not, it’s into the shop for replacement.
In some cars, the brake light switch plunger contacts a plastic or rubber bumper on the brake pedal lever. This bumper can fail, fall off, or break. Without a bumper, the switch in the ON position will activate the brake lights.
If your brake lights won’t turn off, and you find small plastic (or rubber) parts under your feet, you may have this bumper problem.
If that’s the case, you should contact your mechanic to get it repaired. This type of problem is common on cars that have been designed and manufactured by many car brands, including Honda, Hyundai, and Mazda.
#2. Riding Your Brakes.
This is a common problem. While you are driving, you rest your left foot lightly on the brake pedal.
Yes, this value is obvious, as your left foot is available to push on the brakes if you need to stop. But also keep in mind that even the slightest pressure on the brake pedal will travel that pedal a tiny, yet meaningful distance and activate the brake lights.
If your car has an automatic transmission and you commonly use your left foot to brake, you must take care to ensure you do not rest that foot on the brake pedal, or even touch it.
#3. CAN Bus System Failure.
The CAN Bus system can fail. This may be a hardware or software issue and can only be diagnosed and repaired by a qualified mechanic.
Reasons Your Brake Lights Won’t Turn Off (When the Car is Off)
Even if the car isn’t on, the brake lights are capable of functioning. So as you’re shutting everything down, getting out of the car, walking back to the back of your car (to make sure everything is good) YOU notice that your brake lights are on.
How is this possible? Refer to failure modes 2 and 3 in the section above. These conditions that cause the lights to be on while driving will be doing the same when you shut everything down.
How to Check Brake Lights for Proper Operation
There are a few different options for checking brake light operation. And while you may want to trust your clever instrument panel to do it for you, there is nothing like witnessing brake light operation for yourself. Here are a few simple options to do it:
#1. Use a Helper.
The most obvious option is to simply get a friend, family member, neighbor, or passenger to either apply pressure to the brake pedal while you watch, or check the lights while you do the footwork.
#2. Store Front Window Reflection.
While away from home, find a shopping center that has a store up front with parking available directly in front of the store. You can use that storefront window for a mirror. If car and foot traffic permits, back your car into a parking spot directly in front of the glass panel of the store.
Now, look in the rearview mirror and apply the brakes. If all is well, you should see all of the brake lights turn on. You can also check your rear turn signals and rear emergency flashers.
Now, repeat this with the engine and ignition off. If you find any locations with no light operation, you can make plans to fix that later on. With the ignition off, the turn signals will not operate.
#3. Garage Door.
If you are home, you can pull your car into your garage and close the garage door behind you. If it is dark, you will be able to see through your rear view mirror, or side mirrors if your brake lights are showing on the garage door.
You can also do this at a non-open-air parking garage while away from home. Just back into your parking space and check your review mirror for brake lights while you apply pressure to the brakes.
This is also a good way to check other lighting issue such as when your reverse lights do not operate.
#4. DIY Tool.
If at home, and you do not have anyone to help, and you also do not have a garage, you can make a tool to apply pressure and hold down the brake pedal. Here is how to make one:
- Open the driver’s side door. With your left hand, press the brake pedal down about an inch. Measure and write down the distance from the pedal to the front of the driver’s seat. It will be about 18″ to 22″, depending on where your seat is positioned.
- Cut a length of 1” x 2” trim board, or an old broom handle, to that length.
- By pressing down the pedal with this tool and wedging the rear end of the tool between the driver’s seat, you are ready to walk back and check your brake lights. Amazingly, a simple snow brush is about the perfect length for this job.
How to Repair Brake Lights That Won’t Turn Off
#1. Change the Brake Light Switch.
The most typical cause of an inoperative brake light is a defective brake light switch. To correct the problem, you’ll need to replace the switch.
To start, find the brake lamp switch. The switch will be located near the brake pedal. To remove the switch, unplug the wiring connector and note the orientation. Next, remove the defective switch.
Install the new switch in the same orientation as the original and secure the wiring connector. Depress the brake pedal once or twice to test the brake lights. The lights should turn off with the pedal released.
#2. Repair the Wiring.
In some cases, the problem may involve the wiring from the brake light switch to the brake lights. Carefully inspect the wiring for signs of damage (for example, frayed wires or corroded connections).
If damaged wiring is found in the brake light switch wiring, you may need to repair the wiring or replace the entire wiring harness. As mentioned earlier, be sure that all wiring connections are secure and complete your test with the brake lights.
#3. Free the Brake Pedal.
There are rare occasions where the brake pedal itself may be stuck in a slightly depressed position which will cause the brake lights to illuminate perennially.
Check that no obstructions are limiting the brake pedal. Look for items jammed below the pedal or held above the pedal. Confirm that there is no obstruction to the brake pedal and that the pedal and spring will return to original position per design.
If this is not the issue, you should still inspect the switch, as well as the switch’s connections.
#4. Realign the Sensor.
Some vehicles have a more complex method of determining if the brake pedal is depressed: by using a sensor(s) system. If a sensor(s) is misaligning or malfunctioning, the lights may stay on when the brake pedal is disengaged or not pressed.
You will need to locate the sensor and re-position or adjust it so that it accurately detects when the brake pedal is pressed and released. Once completed, test the brake lights again.
#5. Repair the CAN bus System.
Among modern vehicles, the most commonly used lighting method is the CAN bus system. If there is a malfunction within the CAN bus system, it may cause the brake lights to illuminate when the brake pedal does not engage.
Unfortunately, unless you have specific mechanical experience and are armed with a correct service manual outlining CAN bus system malfunctions, this will likely remain an unresolved issue.
What To Do If a Brake Light Is Out?
When one brake light isn’t working, the failure mode is likely a burned out bulb. The bulb can be a standard bulb (filament) or specific to the CAN Bus system and is an LED (Light Emitting Diode) if your vehicle is designed for responding to the CAN Bus.
Replacing by a qualified mechanic is recommended, but a good service manual will include instructions for replacing the bulb yourself.
A less likely failure could be a blown fuse. Check the fuse box either under the hood (and usually has a silvery top) or below the dashboard, typically on the passenger side. Your owner’s manual should indicate the location of the fuse panel and identify which fuse powers the brake lights.
A blown fuse must be replaced with one of the same color and amperage rating displayed on the top of the old fuse. Replacement fuses can be purchased at an auto parts store or a quality hardware store.
If the new fuse blows the first time the brakes are applied, then there is a fault somewhere in the wiring. This is not a repeat; this is an uncorrected short that can damage other parts of the electrical system.
If there is more than one brake light bulb that is not working, the more likely failure is in the electrical system and/or the CAN Bus system. See a qualified mechanic for corrective action.
If You MUST Drive
The best thing is to stay off the road if your brake lights won’t turn off (or won’t turn on). But in those cases where you absolutely must return home or to a repair shop, you’ll want to make the best of it and take the appropriate safety measures.
Hand Signals
Using hand signals is the old-school way to communicate your intentions to others on the road. If you’re turning or stopping, stick your arm out the window. You will need to practice these hand signals, just in case you have to.
Slowing Down
If your brake lights won’t turn off then use a little more caution in your driving style. Slow down. This is especially important when approaching intersections, traffic lights and tight corners. This allows you to minimize the risk of having an accident and allows others drivers time to respond to your vehicle.
Keep in mind that your part will add to the confusion of other drivers on the road, so be conservative until you have the situation resolved.
Preventing Accidents
If your brake lights are stuck on, you can keep yourself and others safe with the following tips:
- Keep an extra distance between you and the vehicle in front of you. By maintaining a greater distance than usual, you will have more time to react to sudden stops or changes in traffic flow.
- If you realize your tail lights are out when driving at night, the best course of action is to pull over to a safe place and have a tow truck bring your vehicle to a garage. Driving in the dark with bad taillights is simply asking for an accident to happen.
- Get your brake lights fixed as soon as possible – don’t delay. Take your vehicle to a mechanic and make the needed repairs right away.