Just like every other part on a car, tail lights can stop working, causing a major safety concern. Not only is this dangerous for others on the road, but you can also get a ticket from the police, or fail your state’s inspection process for registration.
But what does it mean when your brake lights work and at the same time, your tail lamps don’t?
Common Causes for Tail Lights That Don’t Work But Brake Lights Do
#1. Bad Tail Light Fuse.
The first thing to check when facing an inoperational electrical component, especially when it comes to lighting, is the fuse.
A fuse is basically a metal band inside a plastic container designed to break when the current flowing through it becomes too high. This protects other parts of the electrical system from becoming damaged.
There are generally two areas that contain fuses in a car.
- One is a fuse panel inside the cabin of the vehicle. This is normally in the front driver or passenger side, behind a plastic panel underneath the dash. In certain cars, it can be found in the glove box or on the side of the dash with the passenger side door opened all the way.
- The second common area to find fuses is in the main fuse box under the hood. It’s a big black box that has a lot of wires attached to it. Once you take the cover off the top of it, you can access the fuses inside including a turn signal and headlamp relay.
Each fuse controls a different electrical component, and their jobs should be listed on the covering you removed to access the fuses. If not, they can be identified by using the owner’s manual, or by looking up a year, make, model specific diagram online.
Once the fuse that controls your tail lights is identified, it can be tested using a fuse tester, which will light up if the fuse is good.
If it doesn’t light up, the fuse should be replaced with one of the same size and amperage. On certain fuses, you can visually see a break in the metal wire inside a bad fuse.
#2. Bad Tail Light Bulbs.
If the fuses under the hood and inside the cab check out fine, the next culprit to look at are the tail lamp bulbs themselves.
This generally involves removing the tail light cover, although some cars have an access panel where you can reach in and twist the bulb out of the lens.
The best way to identify a bad bulb is to remove it from the socket it is plugged into, and closely inspect the filament.
The filament is the wire inside the bulb that creates the illumination. A broken filament means no illumination and you will need to replace it with a new bulb.
Already changed the light bulb with a new one and it’s still not working? Read on.
#3. Socket Failure.
Sometimes the socket the bulb gets plugged into can go bad. Usually this is caused by corrosion, which is caused my moisture entering the socket. However, it can also end up failing due to poor connection of the wires in the back.
If the bulb and fuse both check out good, check the condition of the socket while you have the bulb out of it. Look for any discoloration, like white, blue or brown, and bent or broken pins.
It is also a good idea at this point to check electrical current with a multimeter at the socket. No electricity reaching the pins means a failure somewhere up the electrical line.
#4. Worn Out Wiring.
If there is no electrical current reaching the socket, and fuses have checked out good, it is very probable there is a damaged or broken wire somewhere along the line.
At this point, you’ll need to obtain a wiring diagram and visually inspect the wires along the tail light circuit for broken wires or cracked insulation.
Be sure to check the body grounds coming off this circuit as well. A dirty, loose, or broken ground wire will cause a failure in delivery of electricity to the sockets as well.
#5. Control Switch Failure.
Another cause for tail lights not turning on is the switch that controls them. The head light switch on the dash can fail as well, and should be checked of everything else has checked out good.
Normally, this switch turns on head lights, tail lights, and parking lights. If all other parts of the tail light circuit are in proper working order, it’s possible this switch has failed. This can be pulled out from the dash and checked with a multimeter to verify it has failed.
#6. Dirty or Bad Ambient Light Sensor.
Many modern cars also have an ambient light sensor on the dash. This allows the vehicle’s computer to automatically turn the head lights and tail lights on and off according to how light or dark it is outside.
Some also turn off daytime running lights on newer vehicles. If both your parking lights and headlights don’t turn on but your brake lights still do, this is the likely culprit.
If this sensor goes bad, or gets too dirty, it cannot tell if it is light or dark outside, and may not turn on the lights. But again, this would affect both the head lights and tail lights, and could be easily tested by manually turning the head light switch to the on position.
#7. Faulty Light Control Module or Body Control Module.
Modern vehicles use electronic modules to control various systems, including lights. The Body Control Module (BCM) or Light Control Module (LCM) on certain cars can sometimes malfunction, causing issues with the tail lights while leaving the brake lights functional.
These modules receive input from switches and sensors, then send signals to activate the appropriate lights. If the module responsible for tail lights malfunctions, it may not send the correct signal to turn them on, even when other lights work normally.
Diagnosing a faulty control module often requires specialized diagnostic equipment. If you’ve checked all other components and can’t find the issue, the problem might lie within one of these modules.
But it’s important to discount all other possible causes as control module failure is fairly rare (and not cheap).
#8. Corroded or Loose Battery Terminals.
While it might seem unrelated, corroded or loose battery terminals can cause various electrical issues, including problems with tail lights.
The battery provides power to all electrical systems in your vehicle, and poor connections can lead to inconsistent power delivery.
Corrosion on battery terminals looks like a white, blue, or greenish powdery substance. This buildup can prevent proper electrical flow. Similarly, if the terminals are loose, they may not maintain consistent contact, leading to intermittent electrical problems.
Check your battery terminals for any signs of corrosion or looseness. If you find corrosion, clean the terminals (follow our directions). Make sure the connections are tight, but be careful not to overtighten them.
How to Test Your Tail Lights
The simplest way to check your tail light and brake light operation is to ask someone else for help. Have a friend, family member, or neighbor either press the brake pedal while you observe the lights, or watch the lights for you while you operate the pedal.
But what if nobody else is around? Here’s how to test tail lights by yourself:
Garage Door or Wall Method:
- Park with the rear of your vehicle backed up close to a garage door (inside or outside), wall, or fence.
- Turn on your headlights.
- Exit the vehicle and check for the red glow of tail lights on the wall.
- Return to the driver’s seat, press the brake pedal, and look for a brighter red reflection.
Reflective Surface Method:
- Find a large window or reflective surface in a parking lot.
- Back up close to it, leaving enough room to exit your vehicle.
- Turn on your headlights and check the reflection for your tail lights.
- Apply the brake pedal and look for the brake lights in the reflection.
Camera Method:
- Set up your smartphone to record video of the rear of your car.
- Start recording, then get in your car.
- Turn on your headlights, press the brake pedal, and activate turn signals.
- Saying out loud when you are pressing the brake pedal also helps.
- Review the video to check all light functions.