How To Troubleshoot & Fix Trailer Lights That Aren’t Working

Are you in a situation where your trailer lights aren’t working? We’re glad you’ve found your way here, because properly functioning trailer lights are vital to your safety on the road. In this post, we guide you through troubleshooting and fixing any trailer lights (including travel trailers) that stop working.

What Do Trailer Lights Do & Why Are They Important?

When we use the term “trailer lights,” we’re referring to the set of lights on all trailers (flatbed, boat, travel trailers, fifth-wheels, etc.) that help other drivers see your trailer and understand your intentions as you drive along. This is why the bulbs and electronics for your trailer lights need to be in optimal working condition at all times.

Not only is this important to your safety and the safety of those traveling around you, but it could also be very important from a legal perspective. If you get in an accident, and it’s determined after the fact that your trailer lights weren’t functioning properly, you could be in extra trouble. By law, your trailer lights must be functioning whenever your trailer is being towed on a public roadway.

All trailers are likely to have some combination of the following lights:

• Left turn
• Right turn
• Brakes
• Tail/running lights
• Reverse/backup lights (these are used in 7-pin trailer connections, but are often left out of 4-pin connections for smaller trailers)

Most trailers and travel trailers have left and right turn signal lights as well as running lights, backup lights, and brake lights. It’s important that all trailer lights are functioning properly whenever the trailer is being towed on a public roadway.

Where Do Trailer Lights Get Their Power From?

Typically, trailer lights get their power from the towing vehicle’s electrical system (i.e., the truck, SUV, or car that’s pulling the trailer). This is usually handled through either a 4-way (running lights, left turn/brake, right turn/brake, and a ground wire) or a 7-way (running lights, left turn/brake, right turn/brake, reverse/backup, brake controller, 12V power, and a ground) plug that connects the wiring in the towing vehicle to that of the trailer.

Let’s examine the most common causes of trailer lights not functioning properly.

Common Causes for Trailer Lights Not Working and How to Troubleshoot Them

There are many issues that can contribute to non-functioning or malfunctioning trailer lights. This list should help you troubleshoot any issues you may encounter with the lights on your trailer or travel trailer. If only one of your trailer lights is out, your investigation and repair should be very easy. However, if all of the lights aren’t functioning properly, you’ll need to do a little bit more digging.

NOTE: newer towing vehicles (like our 2024 GMC Sierra) may come from the factory with built-in trailer towing features. If your vehicle has these, then you may have a trailer light check that you can trigger to test all of the lights. It may also provide warnings about issues that need to be addressed. We use ours every time we hook the trailer up, and each day before towing.

Speaking of which, trailer/travel trailer owners should always do a pre-trip inspection that includes the trailer lights. It’s very easy to do, only takes a few minutes, and it could prevent an accident or an expensive citation for non-working trailer lights. If you have a travel partner, simply have that person stand behind the trailer/travel trailer (preferably in view of one of the towing vehicle’s mirrors) as you test each light – brake lights, running lights, and turn signals to make sure all are functioning properly. If you’re traveling alone, you can either walk back and forth to check each light, returning to the towing vehicle to cycle through each light test… or you can position your cell phone as necessary to capture a video as you illuminate the trailer lights. Then, watch the video to make sure all lights are functioning properly.

The trailer light test screen in our 2024 GMC Sierra 1500. Not only can you trigger a test that cycles between the different lights (so one person can check them all by walking around the truck & trailer), but it also indicates any issues that the system detects (like a failed bulb).

Here are some of the most common reasons for malfunctioning or non-functioning light or lights:

Worn or Blown Bulb

One of the easiest and least complicated issues that result in a non-functioning trailer light is a worn or blown bulb. If this is your problem, the symptom is most likely to be one of your lights failing to illuminate or being extremely dull. This is an issue that is very easy to resolve by simply replacing the bulb with a new one (removing the bulb and bringing it with you to an auto parts store will ensure you get the correct replacement).

Fuses

As we noted in the previous section, most trailer lights get their power from the tow vehicle’s electrical system. For this reason, the trailer lights themselves don’t generally have fuses. However, you can often find a dedicated fuse(s) for your trailer lights in your towing vehicle’s fuse box. In vehicles that don’t have dedicated trailer light wiring, the fuses for your towing vehicle lights will be the ones that apply to the trailer lights.

A pinched, worn, or broken wire can blow a fuse in any electrical system. Because the electrical connector (also called the umbilical) between your towing vehicle and trailer is exposed, it can easily become damaged. Since a fuse is an easy thing to check, it’s a reasonable first step in tracking down trailer light issues. Check to make sure you don’t have a blown fuse. If you do, you may have found the sole issue that can be addressed by simply replacing the fuse (always use a replacement fuse of the same amp rating as the one you’re removing).

Remove the appropriate fuse and check the element inside for signs of a break or burn. If found, replace the fuse with a new one that has the same amp rating as the one you’re removing.

If your fuse/fuses is/are intact and in good condition, you’ll need to trace your wires to check for damage.

Corroded or Disconnected Grounds

Power and a common ground connection are transferred from your tow vehicle to your trailer via the umbilical connection. If that transfer of power or ground is interrupted, your trailer lights won’t work properly. This issue is often overlooked, but can certainly be the source of non-working trailer lights.

Your tow vehicle should have a plug that contains a grounding wire that connects to your trailer’s electrical wiring, or (sometimes) directly to the tongue of your trailer (the metal frame of a trailer/RV is commonly used as the source of ground). This may be a 4-pin trailer plug or a 7-pin trailer plug (for more information, see our post on the 7-pin plug, which includes a section on how to change yours from a 4-pin to a 7-pin plug).

This diagram shows the location and function of each pin of a 7-pin trailer plug.

Start by tracing the ground connection (12V grounding wires are usually white) from your trailer’s umbilical cable (the one that connects it to your towing vehicle at the 4- or 7-pin connection port). It may end in an eyelet that’s mounted to your trailer’s chassis using a bolt. Ensure the bolt makes a secure connection to the metal frame and that there’s no corrosion (more on this below).

It’s also possible for each of your trailer lights to have its own ground wire, depending on your setup. But, in either case, there are several ways in which a ground can cause electrical malfunctions. Let’s examine the two most likely reasons for grounding issues that can cause trailer lights to malfunction.

Corrosion

Corrosion or debris can prevent a ground wire from transferring power. Check to ensure that all ground wires are free of rust, debris, corrosion, and paint. If the ground for your trailer lights is a bolt, you’ll also want to check the hole into which the bolt is inserted to be sure it isn’t laden with rust or anything else that might hinder the transfer of power. You may also need a new bolt.

Once clean of corrosion/rust and reinstalled, coating the exposed metal parts with a light layer of dielectric grease  or battery terminal protector  can help prevent this problem from reoccurring too soon.

Loose or Lost Connection

A loose ground connection can cause trailer lights to flicker or fail to function. A good test of this is to shake or move your trailer. If you find that your trailer lights are flickering, you may have a loose ground (or more than one). Tracing and tightening all connections can resolve your issue completely.

How to Troubleshoot Trailer Lights That Aren’t Working

If your trailer lights aren’t working properly, troubleshooting the issue should be a relatively straightforward process once you know what to look for and how to test a few key components. Let’s take a look.

Check the Bulb(s)

It’s possible (and the easiest-to-solve problem) that you have a burned-out bulb. To check for this, you’ll need to remove the cover for the light that isn’t functioning and check the integrity of the bulb. If you find that you have a blown bulb, simply replace the bulb, retest, and you should be good to go.

Check your bulb(s) for signs of a burned or broken filament (the small, often curly, wire stretched between two others). Sometimes, shaking the bulb will let you hear it rattle against the glass.

Check the Fuse(s)

Another one of the easiest things to check, of course, is any fuses that are associated with your trailer lights. As mentioned in a previous section, since trailer lights typically draw their power from the tow vehicle’s electrical system, you can usually find the fuses for your trailer lights in your tow vehicle’s fuse box. In vehicles that don’t have dedicated trailer light wiring, the fuses for your vehicle lights will also be the fuses that apply to the trailer lights.

But you can also trace the 4-pin or 7-pin connection cable (the umbilical) on your trailer. It likely leads to a small black box mounted somewhere on (or near) the A-frame/tongue of the trailer. There may be additional fuses in there that need to be checked.

Inspect and Test the Wiring Harness/Trailer Plug

Inspect the wiring harness for any obvious signs of damage. Check for crimped, pinched, chewed, or cut wires, as well as any other damage, such as scrapes or tears. Damaged components may need to be replaced.

However, if there’s no obvious damage upon inspection, continue testing your wiring harness by wiggling the connection and watching the lights to see if they flash or flicker on and off. Clean the connections with a bit of electrical contact cleaner . For corrosion or built-up dirt, or debris, use a small wire brush to thoroughly clean the connections.

Additionally, check the integrity of the trailer plug by using a trailer light circuit tester. If your towing vehicle doesn’t include a built-in trailer light check feature, you should carry this type of tester. It’s inexpensive and can save you a world of hassle in tracking down an electrical issue. You just need to ensure that you buy the appropriate type of circuit tester for your trailer plug (4- or 7-way). We like this type of trailer circuit tester because it can be used to test either a 4-pin connector or a 7-pin connector:

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Connect your tester/adapter to your towing vehicle’s 4-prong or 7-pin plug (whichever one your trailer has) and verify that your left and right turn signals function properly. If they do, this indicates that your towing vehicle’s electrical circuit and the components on that end are functioning correctly. However, if the tester receives no signal output, then your tow vehicle is likely to blame, which could indicate that a fuse needs to be replaced or there’s a problem with one of the connection points.

If you don’t have a trailer light circuit tester, you can use a multimeter to check for voltage on each pin. Simply set the multimeter to “V” for “voltage” and, with the vehicle’s engine on, check for voltage between each pin and the ground as you depress the brake, initiate a turn signal, turn on the headlights to illuminate the running lights, and put the vehicle in reverse to check the backup lights.

NOTE: you may need to obtain a diagram online, or look in your owner’s manual, that indicates which pins should have voltage to illuminate a specific light. If you fail to get a reading when testing the various functions, there’s likely an issue with the wiring to that pin.

Check the Ground(s)

If the signal output either fails to function properly after entering it or sends power to the wrong wires, then the grounding location is most likely the cause of the malfunctioning light(s). As we noted earlier, addressing grounding issues is generally a matter of either securing a loose nut, replacing a rusty or corroded bolt(s), or removing rust, dirt, or debris around the ground hole itself using a wire brush or some sandpaper. In rare cases, you may need to drill a new grounding hole.

Following the ground wires in your RV may lead you to a point on the chassis where the wire is attached. Check the tightness of the bolt (it could be loose) and for signs of rust or corrosion.

It’s extremely important to keep your trailer/travel trailer lights in good working condition at all times. Be sure to perform a pre-travel inspection and test all trailer lights before hitting the road. If you find that you have a non-working or malfunctioning trailer light, troubleshoot the lights as noted above, and you should be able to repair the issue and be on your way in no time.

UPDATE! NEW TRAILER TOWING & WEIGHT LIMIT POST, VIDEO & CALCULATOR!

We’re excited to announce that we’ve released a new video and blog post all about Trailer Towing & Weight Limits. We’ve included a FREE towing and weight calculator that we designed to do all the math for you, making sure your rig is properly balanced and within all weight limits. Don’t let a sagging trailer alert everyone in the campground that you’re overweight, and don’t let the dreaded trailer sway ruin your trip (or worse)!

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