/ / / Trailer Brake Lights Not Working When Running Lights are On: 5 Fixes!

Trailer Brake Lights Not Working When Running Lights are On: 5 Fixes!

Are your trailer brake lights not working when running lights are on? Don’t worry, as a lot of people face this issue. This can be due to poor connections, grounding, or wiring. The good news is that you can easily follow some quick steps and fix this problem.

Read on to learn why the trailer brake light stops working, learn nine troubleshooting methods, and what the necessary tools are required. Let’s begin!

5 Main Reasons Behind Trailer Running Lights Not Working!

When it comes to ensuring road safety, functioning lights on brakes are necessary. However, if you find that your trailer lights are not working, it is highly essential to identify the underlying causes promptly. Several reasons that lead to this issue are:

  • Blown Fuse: Electrical overload or short circuit.
  • Faulty Bulbs: Burned out or damaged bulbs.
  • Wiring Issues: Damaged or loose connections.
  • Malfunctioning Switch: Braking light switch failure.
  • Trailer Grounding Problems: Poor ground connection.

Now let’s waste no time and dive into troubleshooting.

Trailer Brake Lights Not Working When Running Lights Are On. 5 Main Reasons Behind Trailer Running Lights Not Working!

9 In-Depth Troubleshooting Methods Explained!

As your trailer gets old, it is very common to encounter problems with brake lights. As we found out some issues leading to their faulty include bad bulbs or corroded sockets. However, these methods are easy to fix. Let’s find out what tools and materials we will need.

Necessary Tools and Materials for Repairing Trailer Brakes Light Wiring

While addressing brake light wiring issues, you will need a few essential tools and materials. This will ensure that your process goes smoothly. The items required are as follows:

Essential Tools Needed

Listed below are the crucial tools needed:

  • Test Light
  • Nut Driver
  • Tow Vehicle Tester
  • Fuses
  • 4-in-1 Screwdriver
  • Pry Bar
  • Wiring Diagram
  • Continuity Tester
  • Jumper Wire

Productive Materials Required

The following are the must-have materials:

  • Trailer Wiring Kit
  • 220-Grit Sandpaper
  • Replacement Light Bulb
  • Electrical Contact Cleaner

Now that we are familiar with the materials we need, let’s move to the troubleshooting part and learn about the 9 fixes we can perform.

Hence, follow the below troubleshooting methods given below:

Fix 1. Activate the Ignition and Engage the Brake Pedal

Starting with our first fix, let’s try resolving the issue by pressing the brake pedal. Follow the below-given steps:

Fix 1. Activate the Ignition and Engage the Brake Pedal
  • First, put the key in and turn it to the “ON” position.
  • Ask someone to help by holding down the brake pedal. Doing this will make the brake lights turn on so you can check if they are working properly.
  • If there is no one to help, use a pry bar tool and put it against the brake pedal.

Fix 2. Diagnose Vehicle Issues with a Tester

You can use a tow vehicle tester tool to check if all the lights on your trailer are working. It is plugged into a trailer wire. Now, connect your trailer to your vehicle and attach the trailer light. Meanwhile, ask someone to stand back and observe if the following trailer lights are working properly:

  • Trailer Running lights
  • Stoplights
  • Blinkers
Fix 2. Diagnose Vehicle Issues with a Tester

After following the above ways, again inspect the trailer braking light connected to your vehicle. However, in case the brake light still appears dim or doesn’t work, then resolve this issue further through:

  • Start by disconnecting the harness and plugging the tester into the receptacle on your vehicle.
  • This tester will quickly let you know if there is a problem with the wiring connecting your vehicle to the trailer.

Fix 3. Clean Connector Plugs with Surety

To make sure your connector plugs are working well, it is important to clean them properly. This will help maintain a strong and secure connection.

Fix 3. Clean Connector Plugs with Surety

For this:

  • Clean the plug and socket using a special cleaning product like WD 40 or CRC 05103 QD.
  • In case of trailer lights still looking dim or unable to turn on, make sure to clean the plug connection completely.
  • Spray an electrical contact cleaner on the connector, and then use a small brush to clean the metal pins making contact.

Fix 4. Look After Trailer Frame Lights with Ground Wiring

First, clean up the spot where the wire from the ground makes contact with the metal frame and fasten it securely.

Fix 4. Look After Trailer Frame Lights with Ground Wiring

Following this:

  • Inspect the ground wire closely because most problems occur there.
  • Remove the screw that connects the ground wire and use sandpaper to scrub the wire’s end gently.
  • Similarly, scrub the area where it attaches to the trailer’s frame.
  • If the ground screw appears rusty or broken, replace it with a fresh one.

Fix 5. Inspect the Fuses

You can use a diagram to understand how the wires are connected.

Fix 5. Inspect the Fuses
  • First, turn off the vehicle and check the fuses to see if they are properly connected.
  • Open the fuse box cover under the hood and look at the instructions on the back of the cover.
  • Look for the faulty fuse in the terminal, and test it using a test light to see if it works.
  • Make sure there is a connection on both sides of the fuse and that the fuse itself is not broken. In many cases, you might need to install a new one.

However, for better assistance, have a look at the following tips:

Tip 1: Look for information related to your vehicle in near-service bulletins. Sometimes, they provide important instructions or recommendations.
Tip 2: When you have to replace fuses often to make the brake lights, trailer running lights, and turn signal work again. This means there could be a problem with the trailer wiring or trailer connector. This issue can cause too much strain on the lights, making them stop working correctly.

For a better understanding, check out this video.

Fix 6. Upgrade Faulty Bulbs

Take out a light bulb by giving it a small turn and put in a new one by turning it the opposite way.

Fix 6. Upgrade Faulty Bulbs
  • If one of your trailer lights is not working (always on, blinking, or stopping), you most likely need to change the bulb.
  • Take off the cover, put in a new bulb, and test it.

If it works, you can continue your journey on a trailer.

Fix 7. Restore Rusty Sockets to their Original Condition

To remove rust/corrosion, use smooth sandpaper and a thin stick to reach narrow areas. If the trailer lights still won’t turn on, there might be rust in the socket at the contact points.

Fix 7. Restore Rusty Sockets to their Original Condition


  • Take a small piece of rough sandpaper and stick it to the end of the stick with hot glue.
  • Clean the points by spinning the stick and moving it back and forth.
  • Apply a little bit of grease on the contact parts and put in the bulb.
  • In case the light is not functioning, inspect the mounting bolts that secure the fix trailer lights. Confirm that they are touching the trailer frame and are free from rust.
  • Should you observe any rust on the bolts, use sandpaper to clean them. Then, retest the trailer lights to see if they are working properly.

Fix 8. Perform a Continuity Test for Electrical Connectivity

You can easily join wires together using clips at the ends, like alligator clips. If the trailer lights on one side of the trailer are not working, the wire may have a problem.

Fix 8. Perform a Continuity Test for Electrical Connectivity
  • To check if the wire is broken, first, look at the color of the wire going into the socket. Then, find the same colored wire on the front connector.
  • Connect one end of a long wire to the trailer lights connector pin.
  • Next, connect the other end to a tester that checks if the wire is connected.
  • Use the tester inside the socket. If the trailer lights don’t turn on, follow the wire and look for places where the covering is missing, or the wire is broken.
  • If you find a break, cut the wire at that point, connect a new wire using soldering, and cover it with heat-shrink tubing.

Fix 9. Upgrade Your Trailer Lights Wiring Setup

If the wires in your system are rusty, it is okay to change the entire wiring system. You can get a new wire bundle for around $20 if you can’t fix the rusty parts. This new bundle has everything you need, like the connector, fix trailer lights, lenses, and instructions. You can usually put it in place within about two hours.

Fix 9. Upgrade Your Trailer Lights Wiring Setup

The Importance of Brake and Turn Signal Systems

The wire that controls your turn signal and brakes works the same way for trailers. It helps you indicate when you’re turning and also when you’re braking. The wiring for the turn signal is divided into left and right signals. This makes sense because you must send a separate signal to each bulb when you want to turn.

Knowing if one or both brake lights are malfunctioning can help determine the exact electrical problem. You can focus on that specific wire after knowing if only one brake light is not working. However, if both brake lights are not working, it usually means there is a general problem, like a bad ground wire connection.

Here is a video explaining turn signals not working.


Why does my trailer have running lights but no brake lights?

If you have working trailer running lights but no turn signals or brake lights, that could be due to the faulty fuses. You must check if they are installed and working properly.

Why do my trailer lights go out when I turn on my headlights?

When your boat trailer lights go out, if you turn on the headlight, it is a classic ground issue. If the ground white wire to the trailer frame or the tail lights is not connected. Securely clean rust and paint-free surfaces. When the headlights are turned, the extra amperage drawn for the trailer running lights will cause lighting issues.

Why are my trailer brakes not working but have power?

In case your trailer brakes are not working but have power, the common causes of this condition are; poor electrical connections, open circuits, and insufficient wire size. Other issues might include broken wires, blown fuse, and improperly functioning controllers or resistors.

How do you activate trailer brakes?

Usually, trailer braking controllers have a button to press to make the brakes work. This lets you use the trailer brakes whenever you want without using the brakes in the vehicle.

How do you test a trailer brake switch?

You can test a trailer’s brake switch by directly applying +12V power from a fully-charged battery. You can apply this power to the hot and ground contacts on the trailer’s plug. Or to an individual brake assembly’s two wires.


Having trailer braking lights that don’t work when the running lights are on can be frustrating. First, inspect the fuse, ensure it’s not blown fuse, and ensure the ground connection is secure. If those things are okay, examine the wiring for any signs of damage or corrosion. Once you identify and correct the problem, your tow vehicle brake lights will be back in action.

Have you ever experienced this issue before? What steps did you take to fix it? We would love to hear about your experiences and any additional tips you may have. Feel free to share your feedback and thoughts in the comments section below.

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