Trailer Lights Not Working: A Troubleshooting Order That Actually Finds It

Every trailer owner eventually hooks up, walks around the back and finds a light that does not work. It is the most common trailer problem there is, and it is also the one people spend the most time on because they start at the wrong end. The fault is almost always at a connection, and there are only a few of them. Work through them in this order and you will usually find it before you need a tool.

First: which lights, and on which side

The pattern tells you where to look. If everything on the trailer is dead, the problem is at the plug, the ground or the truck. If one whole function is out, say running lights work but neither brake light does, the problem is that circuit's pin or wire. If a single fixture is out, the problem is at that fixture. If the lights work but flicker or dim when you turn, it is the ground. Sort out which of those you have before you touch anything.

The plug

Pull the plug and look at both halves. Corrosion on the pins, a bent pin, mud or a wasp nest in the socket, or a plug that is not fully seated accounts for most trailer light problems. Clean the pins with a small wire brush or a pencil eraser, put dielectric grease on them, and plug it in firmly. If the trailer plug hangs loose and drags when unhooked, it has been through puddles and it is where your problem lives. Our wiring and brake controller post has the pin layout for a 7-way connector so you know which pin is which.

The ground

A trailer's lights ground through the frame, and the frame grounds through the white wire at the plug. If that connection is loose, rusty or painted over, the lights look for another path to ground and behave strangely: dimming, flickering, the wrong light coming on when you brake. Find the ground wire's bolt on the tongue, take it off, clean the frame to bare metal, and put it back with a star washer. Do the same at each light fixture if they ground locally. This one fix cures more mystery light problems than everything else combined.

The truck side

Before you go any further into the trailer, prove the truck is sending power. A cheap plug tester from the parts store lights up each function at the truck's socket. If the truck is not putting out a brake signal, the trailer cannot show one, and the fault is a fuse, a relay or the truck's own wiring. Trucks with a factory tow package often have a separate fuse for the trailer circuits, and it is the first thing to check.

The harness and the fixture

Only now go into the trailer. Look along the frame for a wire that has rubbed through, a splice that has pulled apart or a section that got pinched by a jack. On a single dead fixture, pull the lens and look at the bulb or the LED board and the pigtail behind it; on sealed LED fixtures the fixture is the part, and it is replaced as a unit. Make sure water has not been sitting in the fixture, which is common at the rear of a trailer that gets pressure washed.

Keeping it fixed

• Dielectric grease on the plug every few months, and a cap on the trailer plug when it is unhooked.
• The ground bolt cleaned once a year, before hunting season.
• Every light checked every hookup, not just the ones you can see in the mirror.
• Rubbed wires taped and rerouted, not just taped.

LED fixtures on new trailers from our lot fail far less often than the old incandescent ones, which is why they are standard on the builds we sell. If the lights on an older trailer have become a recurring fight, the in-stock units and the options list show what a current lighting package looks like, and our maintenance checklist folds the light check into a routine.