Trailer Lights Not Working On One Side: Quick Fix Guide

Are you having trouble with your trailer lights not working on one side? It can be frustrating and even dangerous to drive with malfunctioning lights.

A wiring issue is the most common reason for a trailer light not working on one side. This can range from a broken wire causing a short or corrosion that hinders current flow. The good news is that trailer lights are simple to diagnose and fix as each light is just a ground wire and hot wire.

In this article, we’ll look at common reasons why your trailer lights might be out on one side. We’ll also discuss troubleshooting steps you can take to identify the issue and potential solutions for fixing it, including:

  • Turn Signal and Brake Lights
  • Running Lights
  • Testing Wires
  • Ground
  • Connector

Whether you have a boat trailer, utility trailer, or travel trailer, these principles apply to fixing your trailer lights. Even if you have LED trailer lights, the wiring and testing methods are the same.

Table of Contents

Turn Signal and Brake Light

First, let’s explore if you have issues with your signal light and trailer brake light on one side. I am putting the turn signal and the brake light under the same category because they share the same wire. So if you have a broken wire, you will likely have a brake light issue and a turn signal issue.

However, the difference is that there are separate wires for the trailer’s left and right sides. So if your left side has an issue, there is a yellow wire that we will be looking into, and if your issue is the right side, then the green wire is where we will start to look.

What’s neat is that these same wire colors and single wire for the turn signal and brake light are true for a 4 pin or 7 pin connectors. So regardless of your pin connector size, we can walk through these tests to see the issue.

Running Light

Running lights, sometimes called tail lights, are wired differently than your turn signal and brake light. Instead of a separate wire running to each side of your trailer, tail lights have one wire that powers both sides. Your 4 pin or 7 pin connector has one connection point for running lights. From there, the wiring harness splits into two to supply both lights.

So if only one side of your running lights is not working, it is likely a bad bulb or connection to the light housing or socket. For more info on tail lights, visit my post on why your trailer running lights are not working.

Testing Wire

When your trailer lights are not working on one side, it is likely an issue with the wire. Since the turn signal and the brake light have one hot wire for the left and one for the right, this gives us our first clue where to start looking for the fix.

Step 1: Visual Inspection of Wire

The first step to finding the issue is to do a visual inspection of your wiring. Trailer wiring usually is exposed, which makes inspection easy.

Inspect for any broken wires.

Start with the color-coded wire for the side you are having trouble with. The green wire controls the right side of the trailer, whereas the left side is the yellow wire.

  • Look for any signs of a broken wire. Since trailer wires are exposed, they are vulnerable to hazards on the road that can cause a wire to be damaged or cut.
  • Inspect wire insulation for cracks. As you inspect your wire, look closely at the sheathing to ensure there are no cracks or rodent damage where a rodent may have chewed. With trailer wires so close to the metal trailer frame, any crack in the insulation will cause a short and for the lights not to work. If you see any cracks, wrap them several times with electrical tape to insulate the copper wire from the trailer frame.
  • Check connections. Check all connections and ensure they are secure with good metal-to-metal contact. Clean any connections that have signs of corrosion. Check waterproof connections to see if any water penetrates the wire. If there are wire nuts, you may want to remove and replace them with a more exterior-grade connector.

Step 2: Check the Fuse

As you were inspecting the wire, did you notice an inline fuse? If so, check to see if you have a blown fuse.

Visually inspect the fuse to see that the metal blade is still intact. You can also double-check with a fuse checker or multimeter. All you need to do is probe the back pins of the fuse and check for continuity. You have continuity if you hear that beep, and the fuse is good. If there is no continuity, you likely have a blown fuse. Replace the fuse and see if that fixes the issue.


The next possible cause for your trailer light not working could be a bad ground wire. Your trailer lights need the hot wire (which we discussed above) and a ground wire.

The ground wire is usually a white wire connected to the trailer frame. Since this wire is completely exposed to the elements, it is more susceptible to corrosion and causing a problem.

Step 3: Check Ground Wire

Checking a ground light is very similar to visually inspecting any wire. Look around for any broken or damaged wire. The main difference between a ground wire vs a hot wire is that the ground wire can touch the trailer without any issues. The main thing is to check for broken wires or signs of corrosion where there is no good metal-to-metal connection.

For more information, visit my post on how to check the ground on trailer wiring, where we go in-depth on checking trailer ground wiring and even using a test light.


The connector or trailer plug is the last possible cause of your trailer light not working. Since this plug connects to your truck or tow vehicle, it is a good idea to inspect to make sure you are getting the connection you need.

As we learned above, whether you have a 4 pin or 7 pin connector, you have a separate wire for the left and the right trailer light. Not getting a good connection with one of those wires at the trailer socket will cause your light on that side not to work.

Step 4: Inspect Trailer Plug

The first step in inspecting a trailer plug is to ensure the metal contacts are clean. A trailer plug is out, exposed to the elements, and easily gets dirty.

Do a visual inspection. If you see any dirt or debris, clean your trailer plug. Also, look at the metal contactor in the plug. Make sure that the metal is clean and you have a good connection.

Final Thoughts

When your trailer light is not working on one side, it is a good sign that you have a wiring issue. By visually inspecting your trailer wire for damage, ensuring secure connections, checking the fuse, examining the ground wire for corrosion, and inspecting the trailer connector for cleanliness and proper connection, you can easily identify and fix your trailer lights.