Tail Light Not Working? How to Find the Problem
Maybe someone flagged you down at a stoplight. Maybe you noticed it in a store window reflection. Either way, you've got a tail light out and you need to figure out what's going on. The good news is it's usually something simple. Here's how to track it down.
Check the Bulb First
Nine times out of ten, it's the bulb. This is the easiest thing to check and the most likely culprit. On most cars, you can access the tail light bulbs from inside the trunk. Pop the trunk lining, twist the bulb socket counterclockwise, and pull it out.
Look at the filament inside the bulb. If it's broken or dark/smoky, that's your answer. Swap in a new one and you're done.
Here's the tricky part. A lot of tail lights use dual-filament bulbs (like a 3157 or 1157). These bulbs have two separate filaments inside. One handles the dim running lights, the other handles the bright brake lights. One filament can burn out while the other still works fine. So your brake lights might be working perfectly while your running lights are dead, or vice versa.
A replacement bulb costs $3 to $10 at any auto parts store. Grab two while you're there. If one side burned out, the other side isn't far behind.
Check the Fuse and Relay
If the bulb looks fine, the next stop is your fuse box. Your owner's manual will tell you which fuse controls the tail lights. On most vehicles, the fuse box is either under the dashboard on the driver's side or under the hood.
Pull the tail light fuse and hold it up to the light. You'll see a thin metal strip inside. If that strip is broken, the fuse is blown. Replace it with one that has the same amp rating. Don't go higher. The fuse is there to protect the wiring.
One thing to keep in mind: the brake light circuit and the running light circuit often have separate fuses. If only your brake lights are out, look for a fuse labeled "stop" or "brake." If it's the running lights, look for one labeled "tail" or "park." They're not always the same fuse.
Brake vs. Running vs. Turn Signal
Your tail light assembly does a lot of jobs at once. Brake lights, running lights, and turn signals are all back there, but they're on separate circuits. That means the fix depends on which function isn't working.
Only brake lights are out? Check the brake light switch. It's a small switch mounted on the brake pedal bracket under the dash. When you press the brake pedal, it pushes against this switch to activate the brake lights. If the switch fails or gets knocked out of position, your brake lights won't come on at all.
Only running lights are out? That's usually the tail light fuse or the bulb filament for running lights. Check both.
Only turn signals are out? The turn signal circuit has its own flasher relay. If your turn signals don't blink at all (not even the dash indicator), the flasher relay is likely dead. It's a small, inexpensive part that plugs into the fuse box or a nearby relay panel.
Socket and Wiring Issues
If you've got a good bulb and the fuse is fine, look at the bulb socket itself. Corroded sockets are really common, especially if you live somewhere with road salt, or if the tail light seal has been letting moisture in.
Pull the bulb out and look inside the socket. If you see green or white crusty buildup on the contacts, that's corrosion. You can clean it with fine sandpaper or electrical contact cleaner. Get the contacts shiny again and try the bulb.
Also check the ground wire. Every tail light assembly has a ground wire that connects to the vehicle's body. If that connection is corroded or loose, you'll get all sorts of weird behavior. Dim lights, flickering, lights that work intermittently. Clean the ground point down to bare metal and make sure the connection is tight.
When to Replace the Whole Assembly
Sometimes the problem isn't the bulb or the fuse. It's the assembly itself. Here are the situations where replacing the whole tail light housing is the right call.
Cracked lens. Once the lens cracks, water gets inside. Water corrodes the sockets, damages the wiring, and fogs up the lens. You can seal a small crack temporarily, but replacement is the real fix.
Damaged circuit board. Some tail light assemblies have a small circuit board inside that routes power to the different bulb sockets. If that board gets corroded or cracked, individual lights will stop working even with good bulbs and fuses.
Failed LED cluster. If your vehicle has factory LED tail lights, the LEDs are built into the assembly. You can't replace individual LEDs. When they start failing (dim spots, partial sections out), the whole assembly gets swapped.
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