Rick's Free Auto Repair Advice

Tail lights not working

Tail lights not working — diagnose and fix

A tail lights not working issue can be caused by a worn out

brake light bulb

Dual filament bulb

dual filament bulb or because of a wiring problem.

Many tail light bulbs are dual filament bulbs

One filament is for the tail lights and the other (brighter) filament is used for the brake lights and turn signals.

Try replacing the dual filament bulb

If only one side isn’t working, chances are that bulb is bad, so try replacing the bulb. While you’re doing that, check for corrosion inside the bulb socket. Look for green corrosion. If you find any, scrub it off with a small nail file. Once clean try the new bulb. If it works, you’ve fixed the problem. But don’t stop there. Prevent further corrosion by applying a light coat of dielectric grease to the terminals in the socket. Find a small packet of dielectric grease at any auto parts store. Apply it with a cotton swap or the tip of a small screwdriver.

If the new tail light bulb doesn’t work, check for power and ground

If the bulb still doesn’t work, check for power in the bulb socket. Depending on the year, make and model of your car or truck, your tail light bulb may have a single or double filament. A single filament bulb is used for parking/tail light only applications and a dual filament bulb is used for parking/tail and turn/stop applications. The car maker uses the brighter filament for the turn and stop functions.

A single filament socket will have only two wires. One is for power, the other for ground.

If you’ve removed the bulb and it only has a single filament (only two wires coming into the socket), use this test procedure. You’ll need and automotive test light or a digital multimeter.

Connect the ground clamp to any clean metal surface. Then turn on the parking lights and probe both terminals in the socket. If there’s power running to the socket, the test light will light. If the test light lights up but the bulb doesn’t work, you either have a defective bulb or you’ve got a faulty ground connection for the socket. Consult a shop manual to find where the ground connection is located.

Test the tail light bulb socket with a digital multimeter

Set the meter to 12-volts DC. Touch the black probe to a clean metal surface. Then probe the two terminals in the socket. You should see +12 volts.

If the test light doesn’t light, power isn’t getting to the socket. If the tail light on the opposite side works, it’s not a fuse problem. Both tail lights are powered by the same fuse for most makes and models. In that case, the problem is either a wiring harness issue or a problem with the body control module (BCM).

In late model vehicles, the headlight switch doesn’t actually switch power to the tail lights. It only acts as an input to the body control module or other smart switching device. In that case, you’ll need a professional diagnostic to solve the problem.

If you have a dual filament tail light bulb

The socket will have three wires. One supplies Tail lights not workingpower to the tail light filament, one supplies power to the turn/stop light filament and the third provides ground. Testing a three wire socket is similar to the two wire socket. With the tail lights turned on, probe all three terminals. Only one should have power. If the test light lights up, the problem is with the new bulb or the ground connection.

If the tail lights go out when you turn on the headlights or any other electrical accessory

This is a sign of a poor ground connection. Get a wiring diagram to locate the point where the tail light ground wire connects to the body. Remove and clean that ground connection and reconnect.

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat

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