How to Test Brake Lights

By: Brian Boone & Talon Homer  | 

car brake lights
You want to make sure your brake lights are always working. Bernhard Lang/Getty Images

It's critical to have working brake lights on your car. They're a safety feature and a tool of courtesy to other drivers. Illuminated brake lights indicate your car is slowing down or has stopped, giving other cars — in daylight, dim light or darkness — plenty of time to stop so they don't crash into you. Aside from all that, if your brake lights don't work you can get a traffic ticket.

But you can avoid all those problems by testing your brake lights once in a while — a couple of times a year is sufficient. It's especially important to do a test before the low-light winter months set in. If your brake lights aren't working, fix them immediately for safety's sake.

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It's simple enough to test your brakes. Just have a friend stand behind the car while you step on the brake pedal and have him or her tell you if everything looks OK. If you're working alone, rig up a mirror or lay a broomstick on the pedal and tuck it into the seat, then walk behind the car and look for yourself. (Obviously, the car needs to be in "park" when you do this.) And don't forget to check all your brake lights. Cars built in recent years feature three brake lights — one on either side in the rear, both of which are implanted in the rear bumper, and a center light in the trunk or back window.

Brake lights, of course, depend on a car's electrical system, which runs on a series of switches and fuses to protect circuits. If one or more of your brake lights aren't working properly, it could mean one of three things: The brake light system fuse is blown, the brake light bulbs are burned out or the brake light wiring switch is broken. All these issues are easy to troubleshoot.

Testing the Brake Light System Fuse

If none of your three brake lights is working, it's doubtful that all the individual bulbs have burned out. It's far more likely you have an electrical system problem. It may sound complicated and expensive, but don't worry. Odds are the electrical system trouble is nothing more than a failed brake light system fuse.

Each part of the car's electrical system corresponds to a fuse, which protects it from amperage overload; if one electrical component blows out, the rest of the car doesn't get fried. If the fuse fails, electricity can't reach the lights, which may be in fine working order otherwise.

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Like all fuses, the brake light system fuse can be found in the power distribution center, which is under the dashboard or tucked away underneath the hood. Never heard of the power distribution center? It's just the technical name for a fuse box. Using your car's manual, locate the fuse that correlates to the brake lights.

You'll need to connect a test light for the next step (you can get one at any auto parts store). Turn your car's ignition to the "on" position, grab the test light and attach it to a ground source, like the dash or the body of the car and gently press the tip of the tester to each of the fuse's two ends. Now, press down lightly on the gas pedal. Does the test light illuminate? If so, the fuse is functional, and the problem most likely is a used-up brake light bulb. If, however, the test light illuminates when the connection is only pressed against one side of the fuse, the fuse is faulty. If the test light fails to light at all, regardless of where it's connected to the fuse, you definitely need to replace the fuse. Make sure the replacement also has the same amp rating as the original fuse.

But wait, you're not done! Once you put in a new fuse, you'll need to test it again. If both lights trigger, the fuse is working and the repair is complete. Be aware that a blown fuse is often an indicator of electrical faults higher up in the system. In such a case, the new fuse will burn out as well and you should consult an electrical technician to pinpoint the exact location of the fault.

If the new fuse doesn't work while you have a foot on the pedal, the circuit itself is shorted out. That's a more complicated and expensive fix, which will need to be handled by a professional mechanic.

If you're lucky, maybe you just need a new light bulb. We'll show you how.

Testing Brake Light Bulbs

Round rear light of a red luxury sports car.
Replacement light bulbs are pretty cheap, unless you are using LED bulbs. Stefan Randholm/Getty Images

Replacing a brake light bulb is a simple task. Generally, you'll go in through the trunk of the car or the fender (check your manual for specifics, especially for the center brake light change-out procedure). First, gently remove any casing or bulb trim that might be present and set it aside. Next, unscrew the bulb as you would any light bulb. Inspect the housing of the bulb, as well as the filament inside. Is the filament blackened and broken? Then you're going to need a new bulb. Is the bulb cracked? You'll need to replace it.

It's important to check the other brake light bulbs as well. Do the self-test (with the broomstick) or get a friend or a large mirror situated so you can see from the driver's seat whether the brake lights go on and when you apply pressure to the brake pedal.

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Replacement bulbs cost about $10 for a basic model and up to $30 for extra bright halogen lights. (They're usually sold in packs of two.) Many newer models also feature LED bulbs at the front and rear, which typically cost more and may require professional installation. Make sure to consult your owner's manual to determine the type of bulb and wattage you'll need for your particular vehicle. Once the new bulbs are in, test them again. If they don't work at that point, you may have a bad socket, which will require the attention of a professional mechanic.

The issue may also lie with the brake light switch. So how do you test it?

Testing the Brake Light Switch

So, you've tested the brake light fuse and the brake light bulbs and there's still no juice flowing to those locations. In this situation, you'll want to check the brake light switch. It's a mechanism that connects and completes the brake light's circuit in the car's electrical system. It's a very basic two-wire switch: One wire controls the power going in while the other wire controls the power going out.

The switch is located near the brake pedal and it's probably marked. Once more, get out your test light and ground it as you did when you checked the fuses. Place the sensor on just one of the two wires and hold the brake pedal down as you do so. Then test the other wire. If power is connected and the switch is working properly, the test bulbs will illuminate. If it doesn't light up, the brake light switch is faulty and will need to be replaced. If your switch is a more complicated setup, consisting of more than two wires, use the owner's manual to locate the primary "power in" and "power out" wires and test those.

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If you do all of these things and still can't get your brake lights to work, your car may have some different systems that need to be checked. For instance, some cars' brake lights and turn indicators are wired together, which means you'll need to inspect that combined system and its fuses. Modern cars feature a dedicated "brake light control module," along with an integrated computer system and onboard diagnostic scans to pinpoint any problems.

Fixing a vehicle's brake lights is usually an easy task, but there are occasions to consult a professional mechanic. Whatever your car requires, it's important to get those lights fixed as soon as possible.

Originally Published: Oct 18, 2010

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