How Do Brake Lights Work?
While brake lights might seem simple, there’s actually a lot that happens between the split-second when you step on the brake pedal, and the lights turn on at the back of the car. This includes both the electrical components of the brake lights and the hydraulic system that operates the physical connection between the brake pads and rotors.
As your foot touches the brake pedal, the pressure triggers the brake light switch to come on. Simultaneously, the brake booster amplifies the physical force sent to the brake master cylinder, which turns it into hydraulic pressure. This engages the brake calipers and pads to start slowing the car.
Meanwhile, the electrical signal created by the brake light switch passes through wires and the brake light relay. A small amount of current is then sent to the brake light fixture at the back of the car to activate the brake lights.
If you take your foot off the brake pedal, it rises, instantly deactivating the brake light switch. The circuit to the brake lights is open, turning them off. At the same time, the hydraulic pressure in the brake lines immediately dissipates, releasing the brake calipers and pads from the disc rotors.
6 Crucial Reasons Why Brake Lights Won’t Turn Off
Brake lights that won’t turn off can be due to something as simple as a bad or improperly aligned brake light switch. This is a relatively cheap fix that most DIY mechanics can handle for less than $50. Unfortunately, the brake lights staying on could also be a more serious short-circuited wire or even a dangerous problem with the brake booster.
1. A Brake Light Switch Problem
If the brake light switch by the brake pedal is misaligned, stuck, or damaged, it can cause the brake lights to stay on regardless of the pedal’s true position. This is one of the cheaper and easier fixes that most capable DIY mechanics can handle independently.
Normally, when you step on the brake pedal, it presses on a small switch with a tiny flexible spring inside a shaft. As the switch is pushed into the housing, metal contacts complete the circuit, which allows electricity to flow to the brake lights, causing them to turn on.
The internal spring pushes the button back when you take your foot off the pedal. This severs the connection between the electrical contacts inside the switch, causing the brake lights to turn off.
While the brake lights staying on could be due to the internal spring getting stuck, the most likely reason is the switch itself being misaligned. All it takes is an accidental bump of your foot, or perhaps you were recently working on something in the driver’s side footwell and mistakenly changed the position of the brake light switch.
How to Diagnose a Brake Light Switch Problem
The two most common signs of a brake light switch problem are the brake lights staying on and the cruise control turning itself off randomly. If the internal spring is faulty, the brake lights might flicker or seem to work correctly sometimes, then stay on at random.
If the brake light switch is misaligned with the pedal, the brake lights will get stuck on, and the cruise control will likely not engage at all. In some cars, this could even cause the ABS warning light to come on.
I checked with several auto parts stores and mechanics to get a ballpark idea of pricing to replace a bad brake light switch.
How to Realign a Brake Light Switch
If your tests revealed that the reason the brake lights won’t turn off is due to a misaligned brake light switch, you might be able to fix it yourself in less than 20 minutes. Don’t be surprised if you realign the switch perfectly, but corroded connections inside still force you to replace the brake light switch.
How to Replace a Bad Brake Light Switch
A bad brake light switch that causes the brake lights to stay on will either need to be realigned to the correct position or replaced.
The part cost for a new brake light switch averages $20 to $45 for most domestic models.
The cost to have a mechanic replace a faulty brake light switch will add another $50 to $75 to the final repair bill, which is around $100.
2. A Bad Brake Light Relay
The next most likely reason why brake lights won’t turn off is a faulty brake light relay. It’s basically a sophisticated fuse for the brake lights and taillights. It can sometimes short out in a way that completes the brake light circuit regardless of input from the brake pedal switch.
How to Diagnose a Bad Brake Light Relay
Brake lights that won’t turn off due to a problem with the brake light relay might also affect other things like the cruise control. Luckily, it’s very easy to check.
The brake light fuse or relay will either be in the interior fuse panel in one of the footwells or in the engine bay’s relay box. Usually, when the brake lights stay on, there is a larger relay in the engine bay. You can find the location in your owner’s manual.
If it looks burned out or corroded, you might be able to get away with replacing it for $20 or less.
QUICK TEST
If you don’t have a multimeter, you can quickly test a suspected bad brake light relay simply by swapping it with another relay with the same rating. If the other device doesn’t work or stays on continuously, but the brake lights act normally again, you’ll know it’s the relay.
3. Short-Circuited Wires
If your brake lights still don’t turn off, even after testing and swapping the relay, you might have a short in the wires leading to the brake light fixtures. This is the next most likely assumption if the brake light relay or fuse burns out again right after you replace it.
How to Diagnose Short-Circuited Wires
Brake lights that stay on due to a short-circuited wire used to be a total guessing game. Back in the day, you could spend an entire afternoon chasing electrical gremlins and cussing out your tools before you managed to find the bad wires.
Thankfully modern science has stepped in to create a simple tool designed to help you find short-circuited wires in a matter of minutes. One usually costs around $25 to $40, and even a novice DIY mechanic can use one by following the instructions on the box.
4. Brake Light Fixture Problems
Several minor problems in the brake light fixtures can also cause the brake lights to stay on even if the brake light switch and the brake fuse/relay are completely fine.
Corrosion or electrical shorts in the brake light connectors are one of the more common fixture-related reasons why brake lights refuse to turn off especially if there’s a crack in the brake light cover or a leak in the housing.
Just a little bit of water getting into the housing can start a condensation cycle that can slowly wreak havoc on the electrical connections and sockets. When the sun shines on the brake lights, just a few drops of water can become steam. Then, it spreads and condenses later in the electronics, leading to long-term rust and corrosion.
How to Diagnose Brake Light Fixture Corrosion
Corrosion can affect the wiring harness, light socket, or brake light connectors, causing them to get stuck on or stay off. The only way to know for sure is to pull the brake light fixture itself.
With some older models, this calls for removing the brake light housing from the outside. Most modern cars have an access port to the trunk or cargo box brake lights.
If there’s no power to the wiring connection, then there’s likely a short or bad connection in the wiring between the brake light fixture and the brake fuse/relay that you missed.
If there’s power at the wiring connection, but the brake light fixture is stuck on or won’t come on again after you clean it, then there’s something wrong inside the brake light fixture. It will need to be replaced.
NOTE
During your brake light fixture inspection, keep a keen eye out for places where water might have gotten in. This includes the seal on the hosing or perhaps a crack in the housing. You’ll need to repair it as well, or you’ll keep having brake light problems.
5. A Bad Turn Signal Switch
An electrical short in the turn signal switch can cause one of the brake lights to stay on. This usually only happens in cars that integrate the turn signal with the rear brake light, but it won’t happen with a car that has the signal and brake light separated.
In a case like this, the switch fuses with the circuit closed. The switch might even click, and the stalk will move back like it normally would when self-canceling—except the single brake light on that side remains on solid or continues to blink.
The other side signal likely won’t come on again. It might also cause problems with the windshield wipers or other accessories controlled on the stalk.
How to Fix a Bad Turn Signal Switch
A bad turn signal switch that causes a brake light to stay on must be replaced. This job is typically within the range of what an average DIY mechanic can handle on their own.
I called around, and the part cost for a new turn signal switch ranges from $25 to $55, with the real-world average for most domestic models being around $35.
The mechanics I spoke to gave me a labor price quote of $35 to $50 to replace the switch for a final price of $60 to $100.
6. Brake Booster Problem
A brake booster problem can cause the brake pedal to crush to the floor or return very slowly, keeping the brake lights from turning off. In a case like this, the brake light switch simply thinks you’re holding the brake pedal down while you’re stopped at a set of lights or gridlock traffic.
The brake booster plays a critical role in amplifying and transmitting the physical force of your foot to the brake master cylinder. From there, it’s translated into hydraulic pressure, which moves through the brake lines to each of the four brake calipers.
The brake booster’s amplification process comes from a vacuum line, vacuum pump, and the interplay of gasket seals with an internal diaphragm. A problem with any one of these components will not only affect the brake lights but also hamper the car’s ability to brake safely.
How to Diagnose a Bad Brake Booster
A bad brake booster that causes the brake lights to get stuck will either leave the brake pedal crushed to the floor or cause it to rise very slowly. This spongy brake pedal will also affect the brake master cylinder’s ability to activate the caliper, causing dangerously long brake distances.
NOTE
I’ve had two cars with brake booster problems in the past. With one it was a problem with the diaphragm and a gasket, which created a strange hissing noise when I stepped on the brake pedal. The other had a bad vacuum line, which didn’t cause any wheezing, but the engine would stumble momentarily when I stepped on the brakes hard.
So, it would help if you watched out for these bad brake booster symptoms. You might get one without the other.
How to Fix a Bad Brake Booster
If your brake lights won’t turn off due to a bad brake booster, you usually have to replace the booster module, the vacuum line, or the vacuum pump. If you’re a capable DIY mechanic, you might be able to do each of these jobs yourself.
However, in my experience, a brake booster problem due to a bad vacuum pump might be better left to the professionals.
The part cost for a new brake booster ranges between $80 to $160, with a real-world average of $120 for most domestic models.
I got some rough price quotes from a few mechanics I know who said the labor cost to replace the brake booster would range from $75 to $125.
If the brake booster itself is in good shape, but you found a leak in the vacuum hose, you might be able to get away with removing and replacing it.
The cost of a new brake booster vacuum hose is usually less than $50.
You should be able to replace it and test the brake system in less than an hour.
Frequently Asked Questions
What is the average lifespan of a brake light switch?
A brake light switch has an average lifespan of around 100,000 miles, with most lasting the life of the car. However, cars in northern states that see a lot of snow and road salt are more prone to wetness and corrosion in the driver’s footwell, which can prematurely cause problems with the brake light switch.
Does a brake inspection test brake lights?
If your brake lights won’t turn off, a brake inspection will find the cause as well as any other brake system faults. You can get one performed just about anywhere that offers tire rotation. It usually costs between $75 to $125 but might be free if you have a dealership service program. It gives you the peace of mind that comes with knowing every part of your car’s braking system is operating safely.
Conclusion
The most likely reason your brake lights won’t turn off is a faulty or misaligned brake light switch, especially if you’ve recently been working on something under the dash and might have accidentally bumped the switch.
While you’re troubleshooting, also take a moment to check the brake light fuse relay. If it’s bad, and you replace it only to have the problem return, you should suspect short-circuited wires or a problem with the brake light fixture.
A brake booster problem could also cause the brake lights to stay stuck on or delay them turning off. This will often crush the brake pedal to the floor or cause it to rise very slowly, leaving the brake light switch on. It will also cause the brakes to perform poorly, a serious road safety hazard.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.