Trailer Lights Not Working When Headlights Are On: 5 Simple Solutions

Having issues with your trailer lights not working? You’re not alone – this is a common problem faced by many trailer owners. In this article, we’ll explore the possible causes of why your trailer’s lights fail to work. We’ll also provide you with 5 easy-to-follow steps to troubleshoot the issue and get your trailer lights back up and running in no time.

Not having trailer lights is a concern as it affects your turn signal, brakes, and trailer lights. Even if you have running lights, your trailer lights should work properly when your headlights are on.

Below we will explore the 5 most common causes of trailer light problems.

Table of Contents

Check Ground

The ground is the most common cause of trailer light failure. With good reason, as the ground wire is usually exposed to the elements and is known to get corroded easily.

Check ground screws for corrosion

The first thing to do here is to inspect the ground connection for any corrosion or damage. Look at the screw connections. If the metal is not shiny or looks like it has corrosion, unscrew the bolt and clean the wire and the connector to the trailer frame.

You should also check if the wire itself is still intact; if there are broken strands or frays along its length, it will need to be replaced with a new one.

How to test Ground

First, you will need to access the wiring harness for your trailer. It typically runs along the frame of the trailer and has multiple connectors coming off of it. Locate the connector that is supplying power to your lights, and check for any loose or corroded wires. If everything looks secure, then you should move on to testing the ground connection.

Using a multimeter, test for continuity between the light’s negative terminal and one of the chassis grounds on your trailer frame. With a good ground connection present, you should get a reading of zero ohms when testing with your multimeter (if there’s no reading at all, then this confirms that there is an issue with either grounding or power supply). You will also get a beep sound from your multimeter if there is continuity.


The next likely source of the issue is a blown fuse.

Fuses are small parts used in electrical circuits to protect them from power surges or overloads. If your trailer’s lights aren’t working, there might be an issue with the fuse, either because it is too weak or because it has been damaged. Check your truck fuse box for any blown fuses associated with your trailer. You may also have fuses at your trailer as well, so check those as well.

Always a good idea to have extra fuses

Sometimes it is difficult to see the fuse blown so you may need a flashlight. You can also do the same testing you did with the ground and the multimeter. At each fuse pin, you should have continuity. If you don’t, that means the fuse is made and needs to be replaced.

Fuses get blown, so keeping extra in your vehicle is always a good idea.

Trailer Connection

This may sound silly, but it has happened to me. Ensure your 7-pin or 4-pin trailer plug is completely pushed into your truck. I often had to give it that extra push in to make a good connection. If that doesn’t work, continue reading below.

Look for any corrosion or damage around the trailer’s electrical connection. If there is any visible sign of wear and tear, clean it off with a wire brush and some sandpaper. Be careful not to jam anything into your connectors or widen the connector opening. A folded piece of sandpaper is all you may need.

Also, you can use a contact cleaner spray. This product is great as it will get to the hard-to-reach spots and is quick-drying, and will not damage your connector or wiring. Another great tip is to use a small pipe cleaner from the dollar store to get inside the connections without damage.

Light Bulb

It may seem obvious, but this is often the root of the problem. If a bulb is blown or loose, replace or tighten it up, and you’ll be good to go! More experienced DIYers may want to take a closer look at these components for potential damage or corrosion.

Inspect light bulbs and their connections

Also, check for any water or moisture in your lens housing or bulb connections. Many trailers and RV light housings can lose their seal and cause moisture to enter and short out the lighting circuit. If you notice some moisture, try to either seal the light housing or add some dielectric grease to the bulb connector.

Wiring Harness

Wiring harnesses can be damaged by several things, such as fraying over time, weather damage, or even rodents. Start by doing a visual inspection of all of your wires and notice any damaged, corroded, or loose connections. Sometimes it can be as slight as a little bit of wire jacket exposed and causing a short to the trailer frame. So get close and get a flashlight to look for any damage.

Disconnect and reconnect any connectors. Inspect them and check for signs of water, dirt, or corrosion. If you see any dirt, use the sandpaper and the contact cleaner mentioned above.

You can always use a test light as well to see if you are getting power at the connection points to see where the break-in power is at.


In conclusion, trailer lights not working can be caused by several issues, ranging from fuses to wiring problems. However, it is possible to fix these problems and keep your trailer lights shining brightly. Check all the components of your trailer lighting system regularly to ensure that your lights function correctly and safely. If you encounter a problem you cannot diagnose or solve, do not hesitate to contact a professional for help.

For more maintenance on your trailer, check out how often to grease your trailer bearings at another one of my posts.