Trailer Brakes Not Working When Running Lights Are On: 5 Easy Fixes

You’re not alone if you have trouble with your trailer brake lights not working when the running lights are on. Many trailers experience this issue due to a lack of a proper connection, ground, or wiring. Fortunately, you can take some simple steps to fix this issue and get your trailer running safely once again. In this article, we’ll discuss the causes of trailer brakes not working when the running lights are on and how to resolve this problem best.

Knowing that your running lights still work, helps us determine the possible issue with the brake lights. With your running lights working, that shows that power is coming from your trailer connector, and there is just a little digging to find out about the brake lights.

Table of Contents

Brake and Turn Signal

Your turn signal and brake wire are the same regarding trailers. This wire serves the same purpose for turning as well as braking.

Also, the turn signal wiring is separated by the left and right signals. It makes sense since you need to send a separate electrical signal to get either bulb working when you want to make a turn.

Trailers use the same brake and turn signal wiring

And knowing if one or both brake lights are out will help determine the electrical issue. When just one of your brakes lights it out, you can isolate the issue to focus on that particular wire. When both your brake lights are out, that usually indicates a universal problem like a bad ground.

5 Easy Fixes

Let’s review the 5 most common causes when brake lights can fail on your trailer. These issues are common for all trailers, including travel trailers and campers. Brake lights for these trailers are all wired very similarly and use a 12-volt system. Also, these common issues apply to either LED or old-school incandescent bulbs. So diving in with the below fixes should work in most situations.

Trailer Connection

A bad trailer connection is a very common problem. Since a trailer connector is exposed to the elements and trying to mate up with the tow vehicle, it can wear out or get corroded. With a worn connector, you may not even be aware as the connector housing itself may seem fine. But the metal connector could be worn and not secure.

Also, corrosion and dirt are other potential causes of a bad connection. Leaving your connector unprotected from the weather and dirt easily causes connection issues.

Use a contact cleaner for a better metal-to-metal connection.

The best thing to do is check your trailer and tow vehicle connectors. Confirm that they are clean and there is shiny metal for a good metal-to-metal connection. Also, check the brass connectors and blades. Make sure none are bent or excessively worn. Be careful not to damage the connectors as you clean them. A chemical contactor cleaner can work great without causing potential damage if there is not too much dirt.

Trailer Wiring

Another potential issue is bad trailer wiring. This issue could be anything from rodent damage to road damage to weather exposure. The next step is visually inspecting the wiring that leads from the brake light. Look for any signs of where a short in the wiring could be. This can be like some rodent damage where the wire jacket was eaten, and the copper wiring was exposed. If this copper wire is exposed even just a little bit and near the frame (which is grounded) it will cause a short and stop the light from working.

Check connections to your trailer lights. Ensure the wire connections are secure and clean for good metal-to-metal connections.


Checking fuses is a common issue that can easily be missed. This simple task should only take a few seconds and can save you much time in aggravation. When checking your fuses, your best bet is not to glance at them and assume they work. Take them out and inspect them. If you have a multimeter, you can check if you have a blown fuse.

To check if a fuse is working correctly, start by turning off the power supply to the circuit. Then remove the fuse from its holder and attach one lead of your multimeter to each end of the fuse. If you see an open reading on your multimeter, it’s time for a new fuse. Be sure to replace it with the same amperage rating as the old one.


Sometimes the issue can be as simple as a bulb. Even if your running lights or tail lights work with the identical bulb, you can have a problem with the brake lights. How? Because these bulbs have a dual filament. So even if one filament works with the tail lights, the second filament for the brake light could be broken.

Brake lights have dual filament

You can visually inspect the light bulb by removing it from the socket and confirming the filaments are intact. Sometimes it is obvious; sometimes, it is not. A couple of ways to check this for sure is either replace with a new bulb or, if just one brake light is out, switch it with the other to see if the problem moves to the different light.

Also, while you get to the bulb, check if there is water in the light trailer housing. After some time, these trailer lights lose their ability to keep water out. And any little water in your light fixture will cause a short and either burn out the light bulb or cause a short in the wiring. Putting in some dielectric grease won’t hurt if you notice any moisture. You can get this at an auto parts store, a clear gel that you put on the light bulb socket. The gel is waterproof and keeps out any water that may get to the bulb. This is a good idea for a boat trailer that gets submerged.


Last but not least is checking the ground wire on the trailer. This is also a very common issue since the ground is needed for every electrical component on the trailer. And with the entire trailer frame as the ground and exposed to weather, it can easily get damaged or corroded.

The ground wire is the white wire connected to your trailer frame and the light fixture. This ground wire is associated with a screw at your trailer frame, with the other wire going right to the brake light. Check the connection to the frame and the wire connection to the light fixture.

I have much more info on how to do this with another post on checking for ground on your trailer. In that post, you will learn how to inspect and verify the ground is working with a test light.

Final Thoughts

Trailer brake lights not working can be a frustrating experience. However, the solution is usually easy to diagnose and repair. We covered the 5 common solutions to this problem. Check for a blown fuse or loose ground first. If those are fine, inspect the wiring for any signs of fraying or corrosion. Once you have identified and corrected the issue, your trailer brake lights will work again in no time. With just a little effort, your trailer will be ready to take on any task confidently and safely.