Tail light bulb doesn’t work
Diagnose and fix a tail light bulb doesn’t work condition
Many times DIYers will have a tail light that doesn’t work and they replace the bulb with a new one. Then they find the new tail light bulb doesn’t work and wonder what to do next. Here are the steps to follow to diagnose the condition.
First, check for corrosion and power at the tail light bulb socket
The most common cause of tail light problems is corrosion inside the tail light bulb socket. With the bulb out, 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 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 
power 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.
©, 2019 Rick Muscoplat
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