Why Are My Brake Lights Not Working?

You've just been told that your brake lights aren't working. Often times it’s by a fellow motorist, but sometimes, it's from an officer in a traffic stop, which is never a fun experience. This is a problem that has safety and legal implications, so ensuring that you have brake lights that work is crucial for a myriad of reasons. Understanding how they work and how to troubleshoot is equally as important.

On most vehicles on the road today, the brake lights still function the way they have for years - via a switch mounted to the brake pedal or a hydraulic switch located on a brake line or the master cylinder itself. A 12V power source, which is protected by a fuse, sits on one side of this switch. The other side is where the power gets sent out to the brake lights after you press down on the brake pedal, which triggers the circuit to close and the brake light bulbs to illuminate.

Many newer vehicles have a brake light switch circuit that is connected to something called the body control module, or BCM for short. The BCM is delivered the information to turn the brake lights on when the circuit is closed or grounded (after you press down on the brake pedal). The BCM additionally shares the switch's status to other computer modules in your car, creating a network of relayed data.

These features need to always be functioning, but as we all know, cars' components can go bad on a whim. To avoid getting a ticket or getting into an accident, you can troubleshoot brake light issues on your own before taking the car to a mechanic. Let's first go over common causes of brake light failure and what you can do to try to fix the problem.

Why are My Brake Lights Staying On or Off?

There are a number of reasons why your brakes lights won’t turn off or on. Some of the most common reasons that your brake lights are out or malfunctioning include:

  • Problems with the brake light switch
  • Issues with the brake light switch circuit – usually a ground
  • Brake light bulbs that have burned out
  • Brake light socket has gone bad or are corroded
  • Turn signal switch defective

The brake light bulb is the first thing you should check since it is the easiest and cheapest repair. If that is not the issue, you are going to need to know how to troubleshoot problems with either the brake light switch itself or its circuit – something that is much easier said than done. At this point, if you feel you need a mechanic to troubleshoot the circuit, feel free to check out our list of Preferred Shops in your area that can help fix your issue.

How to Troubleshoot Brake Lights That Aren’t Working

In order to properly troubleshoot brake lights, the first and most important piece of equipment you’ll need is a Multimeter (Volt Meter) and/or a Voltage Light (probe). Either will do the trick, but the Multimeter will usually prove to be better for testing grounds and continuity. If you’re uncertain how to properly use a Multimeter, you’ll want to tackle that first, as the rest of the article assumes proper use of the equipment.

As mentioned before, start by checking the bulb and any fuses marked brake lights / turn signal / tail lights. In many vehicles, your turn signal bulb’s filament is shared by the brake light as well. This means that when your brake pedal is depressed, this filament lights up. If the brake pedal is pressed when the turn signal is engaged, the turn signal still has to turn this already illuminated bulb on and off, no different than if the brake pedal wasn’t pressed. In most vehicles, the brake light circuit has a relationship with the turn signal circuit.

Once the bulb has been ruled out, your next step is to check the socket. Depending on the style of socket, look for corrosion, melted plastic, and make sure that the socket is completely clean. Many sockets will have dielectric grease (commonly called bulb grease) in them – this is a good thing for keeping pesky corrosion at bay. For the sake of diagnosis though, you will need to take a Q-tip and clean out as much of the grease as possible. Many brake light issues can be traced to a bad socket, and visual inspection is usually the first step to diagnosing this kind of issue. Clean the socket if it is dirty with a micro file, sand paper, wire brush, or electrical solvent, and test the bulb again.

If there’s still an issue, the next step is to test voltage and ground. At this point, it’s a good idea to source a wiring diagram that will show you which wires at the taillights are responsible for your 12V power to the brake lights, and where the ground points are located. Some grounds are right at the taillight assembly, while others share a common ground in a location, like in the trunk or underbody of the vehicle.

Testing voltage and ground involves using the Multimeter to probe the socket and wires, starting at the rear of the car where most problems occur. The lights at the rear of the car usually share a common ground, or series of grounds, and often times these can be the culprit due to corrosion or being dirty.

Start by finding a reliable ground on the body or in the trunk of the vehicle. This could be a clean, exposed bolt, or clean area of the body behind one of the taillights. For reference, the entire body of the vehicle acts as a ground, so once you find a good piece of exposed body metal, you should have a good ground source. Put your multimeter into DC volts, and have a partner press the brake pedal on command. 

Begin by testing the socket pins that engage the brake light filament. If this socket is a dual-pin, the other filament is your driving lights, and these can be easily found by simply turning on the lights of the vehicle to identify. If no voltage is found at the socket, trace the 12V wire from the socket that’s been identified as the brake light wire back and test an area of the wire. This can be done with a probing pin on your multimeter to essentially “stab” the wire. You are doing this to rule out that something in the socket connection is bad. If you suddenly have 12V when you stab the wire, you know your problem is either in the socket or ground. Next, let’s test the ground.

How to Test Bad Ground Connections

Grounds are a constant nemesis on anything with 12V DC power. Just a slight amount of corrosion over time, and suddenly a ground will not function properly, leaving an open circuit. To test grounds, the easiest solution is to put your voltmeter into Continuity (Ohms) setting. Most meters will have an audible diode test setting, which means when the two probes are touched together, you get an audible beep out of the meter, which is far easier than staring at the screen. This setting is ideal for testing that these grounds are working properly with good connection.

Touch one of the probes to your ground bolt you identified in the earlier test. Now, test all the ground connections by touching the probe to the metal connecting lug or terminals as close to the grounding bolt or area as possible. If you hear a beep, you know the ground is good. Even with this, it’s a good idea to loosen the ground bolt, clean the terminal, and re-install. When testing grounds, if you don’t hear the beep, you know that you are not getting continuity from your good, known ground point to the ground being tested, making this testing ground spot suspect. Test grounding locations or wires all the way up to the socket to ensure your ground is still solid.

Troubleshoot the Circuit Further

If all the grounds at the rear of the vehicle check out ok, and you still have no voltage at any of the 12V sources here, your problem is further upstream. Wiring issues further up can also lead to this type of problem. If a wire has shorted or broken, you’ll need to trace that wire all the way back to the dash and brake light switch.

BCM problems can lead to brake lights failing as well in cars that use the BCM as a trigger. A mechanic should check to make sure the circuit is not broken. If it is intact, then the mechanic will need to either look at an available technical service manual or replace the BCM itself.

Brake Light Switch or Turn Signal Problems

Many times, the brake light switch is the culprit. There are several potential causes for it going bad and leading to brake light failure. The switch is activated by the pressing of the pedal, and can get stuck in the OFF or ON position, leaving you with no brake lights or constant brake lights. Inspect the connection to the switch and make sure there is no corrosion or damage, and then, a replacement switch can be installed, and purchased at your local AutoZone. Sometimes the switch can be adjusted so that less pedal travel turns the switch on or off, so keep this in mind as well.

On older vehicles, like classic cars, the brake light switch is often a hydraulic switch, mounted right in the brake fluid circuit – usually connecting to the brake line or into the master cylinder. When brake fluid pressure increases (because the brakes are engaged) this triggers the switch. This switch will have 2 bullet-style connectors on it that are prone to corrosion. Clean these accordingly first, then look into replacing the switch.

As mentioned before, the brake circuit usually travels a path through the turn signal switch. Although uncommon, some vehicles can have a failed turn signal switch that will still give you turn signals, but cause your brake lights not to properly function. When troubleshooting is leading you to dead ends, don’t overlook this switch as a possibility.

Troubleshoot anything electrical can be challenging, but checking off each box in diagnosis will help you narrow down all the potential causes. If you need any parts or advice on bulbs or electrical, stop in your local AutoZone today!

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