You may have experienced this: either your brake lights work while your taillights do not, or your taillights work while your brake lights stop functioning. A common assumption most drivers make is that both lights use the same circuit and bulb. Often, they don't. Brake lights and taillights usually run on separate electrical circuits and in some vehicles, separate bulbs as well. That is why one can work while the other does not.
Before we get into the causes and fixes, we need to understand the difference between brake lights and taillights and what controls each lighting system. Once this is clear, it becomes easier to understand why one of these lights fails and how to diagnose and fix the problem correctly.
Related: Different types of car lights and what each does
Understanding the Difference Between Brake Lights and Taillights
The main difference between brake lights and taillights lies in their jobs and how they are controlled.
Brake lights light up the moment you press the brake pedal. Their primary purpose is to signal to drivers behind you that you are slowing down or coming to a stop. This helps prevent rear-end collisions, especially when you need to stop suddenly or unexpectedly. It is a critical safety feature in all vehicles.
Taillights are connected to your vehicle's headlights and turn on automatically when you switch on the headlights or parking lights. They provide a dimmer, more consistent light than brake lights and remain on as long as the headlights are turned on. Their purpose is to alert other drivers to your vehicle's presence, especially in low-light conditions such as nighttime, fog, rain, or snow.
By improving your visibility on the road, taillights play a vital role in preventing accidents. They help drivers approaching from behind see your vehicle clearly and keep a safe distance.
Both taillights and brake lights are controlled by different triggers and, in most vehicles, separate electrical circuits. Brake lights rely on components such as the brake light switch and their own circuit, while taillights operate through the vehicle's exterior lighting circuit and, in some cases, a relay.
In some vehicles, both functions share the same bulb but use separate filaments and different wiring logic. That is why one lighting function may fail while the other continues working normally.
Why Brake Lights Are Not Working, But Taillights Are
There's nothing more frustrating than realizing your brake lights are not working, but your taillights are. You may think they depend on each other, but that is often not the case. Here are the most common reasons behind this scenario.
1. Burned-Out Brake Light Bulb
The LED brake light bulbs themselves may be faulty. Over time, bulbs can blow out or fail, causing the brake lights to stop working even when the taillights are functioning correctly.
2. Fuse Box and Light Switch Problems
The brake lights might not be working because of issues with the wiring connected to the fuse box and brake light switch. A damaged or loose wire between the bulb socket and the fuse box can disrupt the connection and prevent the brake lights from illuminating.
3. Bad Electrical Ground Wiring
The ground wiring for the brake lights might be damaged, corroded, or loose. This can prevent the brake lights from receiving the power they need to function.
4. Bad Bulb Socket
The socket that holds the brake light bulb can wear out or become damaged. A bad socket may cause the brake lights to fail while the taillights continue to work. Replacing the socket usually fixes this.
5. Turn Signal Switch
In some vehicles, the brake light circuit is integrated with the turn signal circuit through a shared wire. A faulty turn signal switch can therefore cause the brake lights to stop working.
6. High-Mounted (Third) Brake Light Out
The high-mounted or third brake light, which is of quite an importance, is usually a separate bulb or LED strip light on its own circuit. It can fail on its own while your two main brake lights still work, so if only the center light is dark, start there.
Addressing these issues promptly helps ensure your brake lights work correctly and keeps you safe on the road.
Why Taillights Are Not Working, But Brake Lights Are
What if you're facing the opposite situation, like taillights not working while brake lights are? Here are some possible reasons your taillights may stop working even though your brake lights still function.
1. Faulty Fuse
One of the first steps in troubleshooting is to check the taillight fuse. A blown fuse is a common culprit behind non-working taillights.
2. Wiring Issues
As with brake light problems, faulty wiring can affect taillights. Loose, frayed, or damaged wiring in the taillight system can prevent them from working, even though the brake lights, which use a different wiring system, remain functional.
3. Bad Bulbs
The taillight bulbs themselves may be faulty. If the filament inside the bulb is broken, the taillights will not illuminate.
4. Faulty Light Switch
The headlight switch, which also controls the taillights, could be malfunctioning. A faulty switch may prevent the taillights from turning on even when the headlights are activated.
5. Corroded or Loose Sockets
The sockets that hold the taillight bulbs can corrode or loosen over time. This can interrupt the electrical connection and prevent the taillights from working properly.
6. Defective Taillight Relay
Some vehicles use a relay to control power to the taillights. If this relay is defective, it can prevent the taillights from functioning.
7. Grounding Issues
Poor or corroded ground connections can impair taillight operation. Cleaning and securing the grounding points often resolves the issue.
8. Faulty Control Module
In modern vehicles, a control module manages various lighting functions. A malfunctioning module can cause the taillights to fail even when the brake lights are working fine.
When your brake lights or taillights malfunction, it is important to diagnose and fix the issue promptly to ensure safety on the road.
How to Fix Non-Working Brake Lights or Taillights
When your brake lights stop working but your taillights still function, or vice versa, the problem is usually related to a specific part of the lighting circuit. Because both systems use separate circuits, the fix depends on whether the issue is with the bulb, power supply, switch, or wiring connection.
Step 1: Check the Bulb
If your brake lights work but your taillights do not (or vice versa), inspect the bulb first. Remove the bulb and check the filaments for breaks, dark spots, or any signs of damage. You can also test the bulb with a multimeter in continuity mode.
Replace the brake or taillight bulb if the filament inside is damaged. Installing a new bulb usually restores the failed light immediately if there is no other electrical problem.
If the light still does not work after a known-good bulb is fitted, the cause is likely elsewhere: a blown fuse, a faulty brake light switch, a corroded socket, or damaged wiring.
Also Read: DIY Guide to Installing 7443 Brake Lights
Step 2: Check the Fuse
A blown fuse is another major reason a brake light or taillight stops working. Locate the fuse box and identify the correct fuse using your owner's manual. Remove the fuse and inspect it for a broken metal strip or test it with a multimeter for continuity. If the fuse is good, the multimeter beeps; if it is blown, you get no sound or an "OL" reading.
Replace a blown fuse with one of the same amperage rating. If the new fuse blows again shortly afterward, inspect the wiring for a short circuit before installing another replacement.
Step 3: Test the Light Switch
If the brake lights have stopped working while the taillights still function, check the brake light switch located behind the brake pedal.
With the switch disconnected, set your multimeter to continuity mode and probe its terminals while pressing the pedal: a good switch reads near 0 Ω (closed) when pressed and OL (open) when released. To test it in place instead, set the multimeter to DC voltage and confirm the switch passes power downstream when the pedal is pressed.
If the brake lights work but the taillights do not, turn on the parking lights to confirm whether the headlight or parking light switch is supplying power to the taillight circuit. Replace the switch if it fails to send power to the affected circuit.
Step 4: Inspect Wiring and Ground
If the bulb, fuse, and switch are all good but the light still won't work, inspect the wiring and ground connections near the rear light assembly. Look for damaged insulation, loose connectors, corrosion, or broken wires.
Use a multimeter to verify that both power and ground are reaching the affected socket. Repair damaged wiring, clean corroded terminals, and tighten loose ground connections.
Step 5: Test the Relay or Control Module
Relays and body control modules (BCMs) are common in modern vehicles. Use a multimeter to check whether the relay or BCM is sending power to the affected lighting circuit. If the relay fails to energize or the BCM is not supplying power to the brake light or taillight circuit, repair or replace the faulty component.
Should You Drive With Bad Brake Lights or Taillights?
Brake lights and taillights tell other drivers what you are doing on the road — slowing down, stopping, or turning. Driving with a faulty brake light or taillight is risky even if the car feels fine.
A broken brake light can keep the drivers behind you from noticing when you stop, raising the risk of a rear-end collision. A faulty taillight reduces your visibility, especially at night or in bad weather, making your car easier to miss.
You might be able to drive short distances, but you could also get pulled over or fined in many places. The safer move is to fix it as soon as possible rather than putting it off.
Choosing the Right Replacement Brake Light Bulbs
When it comes to automotive lighting, reliability is paramount. Whether you're upgrading your vehicle or replacing a malfunctioning light, these two customer favorites are direct, easy replacements:
- 7443 Red Brake Lights: A top customer choice and a direct replacement for rear brake lights and turn signals. They are ultra-bright, long-lasting, and feature a cooling fan to prevent overheating, with no hyper-flashing or error codes.
- 3157 Red Brake Lights: Another popular option offering brilliant, long-lasting performance. They also feature a cooling fan and are free from hyper-flashing or error codes, with a deep red LED output that covers 360 degrees for maximum visibility.
Conclusion
Brake lights and taillights are powered by separate circuits, which is why one can stop working while the other still functions. Once you know that, diagnosing the problem is mostly a matter of isolating where the circuit breaks down: the bulb, the fuse, the switch, the wiring, or the ground. In most cases, the fix is straightforward and can be handled at home without much hassle.
If it turns out you need a replacement, a quick look at Underground Lighting's bulb range can get you back on the road quickly and safely.