Taillights come on with your headlights and are meant to improve your visibility in poor conditions. They make it easier for motorists driving behind you to see where you are in rain, snow, or in the dark of night. Most modern cars have brake lights integrated with the taillights to come on the instant you step on the brake pedal.
Yet there are times when things like wiring faults, bad fuses, and other electrical gremlins can cause the brake lights to work, even though the taillights don’t.
Having tail lights that do not illuminate, while brake lights remain functional, can be a very dangerous situation if you’re driving at night, or in bad weather. To get the taillights to come on and the brake lights to work normally, you need to know where to look for a problem.
So, let’s dive in and restore the sparkle to your taillights!
How Do Taillights and Brake Lights Work?
Taillights are connected to the main control switch that also operates the headlights, parking lights, and/or running lights. When they’re working properly, the taillights will come on with the headlights.
The brake lights, on the other hand, are connected to the brake light switch, which is directly connected to the brake pedal. Anytime you step on the brake pedal it automatically turns on all the rear brake lights.
Most, but not all, modern cars have the brake and taillights running through the same fuse in a circuit panel under the dash or inside the engine bay. The wiring then runs back to the rear lightbulbs themselves which typically have a dual filament inside. One lower-lumen filament is for the taillights, with the higher-lumen filament for the brake lights.
Though there are some cars, especially older ones, that have a separate fuse and a separate single filament bulb for each brake light and tail light. If you’re not sure how your car’s lights are configured, you can always check your owner’s manual.
6 Reasons Why Your Brake Lights Work but the Tail Lights Don’t
For something so seemingly simple there are a lot of little electrical gremlins that can cause a divorce between your taillights and your brake lights. To get them both back in a happy safety light marriage, you should start by looking at some of the following things.
1. Interior Fuse That Controls The Tail Lights Has Blown
A blown fuse in the cab’s interior fuse box is one of the more common reasons why taillights won’t come on properly while brake lights do. A lot of cars have a cab fuse box either under the driver’s steering column or the passenger seat footwell. You can usually tell which fuse runs your taillights in the owner’s manual.
If you pull it out and you see a discolored splotch in the middle of the plastic housing or the little metal V inside the fuse is broken, you’ll have found the culprit. Simply replace it with exactly the amp-rating automotive fuse. Then turn on the headlights, to see if the taillights also light up again.
2. The Engine Bay Fuse Box
If your car doesn’t have a tail and headlights fuse in the interior fuse panel, then you might need to check under the hood. A lot of older vehicles and some pickup trucks have fuses for the tail and brake lights running through the master fuse box in the engine bay. This is even more likely to be the case if your vehicle has a towing package wiring harness.
Sometimes an electrical surge from the battery or alternator or a wiring short can cause the fuse to blow. Though with these fuses, it might not always be obvious if it’s burned out. Especially if the fuse is covered by a relay-style housing.
Though it’s easy enough to test with a simple battery tester light or a multimeter. If the test light comes on or the multimeter gives you a reading then the fuse is good and the reason why your taillights don’t work but brake lights are relies elsewhere.
3. Loose, Frayed, Or Otherwise Faulty Tail Light Wiring
If you find and replace a bad taillight fuse only to have it burn out again a little while later, then you likely have a bad wire somewhere between the fuse box and the taillights. This can be a fickle problem to try to diagnose if your make and model runs the wiring harness through a conduit installed inside the frame of the vehicle.
The most likely places to find this shorted or damaged tail light wire is in the driver’s footwell near the brake pedal or in the rear tail light housing. Some older vehicles will run under the step plate of the driver’s side door. You can remove the plastic panel to check if something in the wiring harness has been damaged.
If you find a bare, damaged wire or a wire with a melted protective coating, then chances are good this is the gremlin causing your fuse to burn out and preventing your taillights from working despite the brake lights working fine. If you’re lucky and the bad taillight wire is someplace that’s easy to access you might be able to simply patch, pigtail or replace the bad section of wire yourself.
If the bad section of wire is in a difficult-to-reach place, such as the conduit that passes through the door or the door well, you could end up paying a lot of money in labor time to have a mechanic ferret it out.
Having a mechanic run all new wires for the taillights and update your wiring harness can be as little as $120 to $300. Though a $500 repair bill isn’t out of the realm of possibility.
4. Faulty Headlight Control Switch
The contact leads in the headlight control switch can wear out which causes the taillights to not come on, even if your brake lights do. This is because brake lights are turned on by the brake light switch connected to the brake pedal.
If it is the headlight control switch that’s preventing your taillights from coming on, then you most likely wouldn’t have headlights or running lights either. If this is the case, then chances are good the control switch is at fault.
With a little prying and some careful jimmying, you should be able to get the headlight control switch out. You can then test the contact leads with a multimeter to confirm or eliminate it as the reason why your taillights don’t work even though your brake lights do.
Replacing a headlight control switch usually costs around $100 to $150 for the switch and to have a mechanic update the wiring.
5. Bad or Failing Tail Lamp Bulb
A lot of modern vehicles have a dual filament taillight bulb with a separate filament for the taillights and one for the brake lights. The taillight filament can burn out or be damaged, but the brake light filament might still be fine, leading to a situation where the taillights don’t work but the brake lights do. This can happen with age, or if water manages to get inside the taillight housing.
There are also some modern cars and a few older vehicles that have separate bulbs for the brake and taillights. Then the two bulbs merely sit together in the same housing.
To access and test the taillight filament, follow these steps:
The cost for a replacement dual-filament taillight and brake light bulb is usually less than $50.
6: Corrosion in Taillight Connections
While you’re working with the taillights also make sure to also check for corrosion on the connections. Humidity or moisture that gets into the taillight housing can gradually cause corrosion on the leads coming out of the wiring harness, which can result in the taillights ceasing to function.
If you see anything green or white on the connections leading out of the wiring harness conduit, take the time to clean it with a little baking soda and water paste, and an old toothbrush. Then reconnected the taillight bulb and test the lights again.
Frequently Asked Questions
Can A Dusty Ambient Light Sensor Prevent My Taillights From Coming On?
A lot of newer cars with automatic headlights and taillights have some sort of ambient light sensor on the dash or mounted to the windshield. If it’s dirty, or dusty, the sensor would read the ambient light as being dimmer than what it is, and it would automatically turn the head and taillights on.
If the sensor was damaged by something like dropping a heavy purse on it or an accidental bird strike on the highway, then it might not work correctly to automatically turn the head and taillights on automatically. Though the taillights would still come on when you turn the headlights on manually via the control switch on the dash or the indicator stalk.
Is It Illegal to Drive Without Taillights?
Technically, you have to have at least one functional taillight for your car to be considered road legal. If both your taillights are out, but you have a central bar taillight in the rear window, most states will still consider you to be road legal. This will at least buy you some time to figure out why your taillight doesn’t come on even though your brake lights do.
Can I Use Portable Taillights
Magnetic portable taillights are road legal, and you can connect them to your rear towing electrical harness if you have one. If they work correctly, then it can also help you determine if and where a potential wiring short is between your conventional taillights and the fuse box.
If your brake lights are coming on but your taillights don’t, it’s most likely a burned-out fuse or a bad element in the dual filament taillight bulb. The fuses for the taillights are usually near the driver’s steering column or the passenger side footwell. Though they could be in the engine bay fuse box with the other relays. Checking them is a logical easy first step in diagnosing the problem.
Also, pay attention to the headlights and running lights. If they too aren’t coming on, then it could be the control switch on the dash or the indicator stalk. This can be tested via a multimeter and replaced if necessary.
If you replace a bad taillight fuse and it burns out again, then you should suspect a shorted-out wire in the driver’s side footwell, the door threshold panel, or in the wiring harness in the taillights. Most of the time the bad section of wire needs to be replaced. Though before you do that, check the taillight bulb itself.
You can usually go through a panel in the trunk to pull the taillight bulb and the connection to the wiring harness out. Look for signs of corrosion or shorted wires. Then gently test the dual filament bulb with a test light. If the taillight filament is burned out, you’ll need to completely replace the entire bulb.
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.