Trailer Running Lights Not Working

Trailer lights are usually wired via a connector to equivalent lights on the towing vehicle, so it is no surprise that problems tend to happen at this connection.

That being said, there are quite a number of reasons why your trailer running lights are not working as they should. This article aims to highlight the necessary issues you could be facing as well as any solutions to those particular problems.

The connector used is a four-terminal one which provides tail, right brake and turn, ground return, and a left turn and brake signal. Some larger connectors, particularly on larger travel trailers come with electric brakes, 12V accessories port, and reverse lights.

The majority of wiring problems faced tending to begin right at the connectors. They could also be at the point that joins the light to the connect. Certain problem areas such as the location of the wiring flexes can be quite careless.

The following will show you how to recognize and fix your trailer lights. The good thing is that the majority of these fixes are easy and quick. Some tools will be required of you. They are:

  • Continuity tester
  • 4 in 1 screwdriver
  • Jumper wire
  • Tow vehicle tester
  • Nut wire 
  • Electrical contact cleaner
  • 220 grit sandpaper
  • Wiring kit
  • Replacement bulb

If your trailer has begun to show that it is aging, then there is a high chance that you have had issues with the running lights. Running lights have to be functioning properly as this is a prerequisite to operate the trailer safely.

It is for this reason that you have to check your lights frequently. there are some issues like having a bad running light ground or a burnt bulb that can be quite easy to solve. there are also other problems that can be quite difficult you recognize and solve. Read on to discover the issues you might have and the particular fixes to those issues.

Troubleshooting the trailer’s running lights using a tester

The first thing you should do is to check your vehicle’s system. Ensure that your tow vehicle tester is inserted correctly in the vehicle. Doing this can enable you to know f the trailer’s running lighting system is correctly functioning.

There are two kinds of wiring testers, you have to check the configuration of the vehicle’s plug. Take your time to hook up the trailer as you would normally do and connecting the light harness of the trailer.

This part will involve having someone else around as they would have to check to see if the trainer’s running lights, blinkers, and stoplights are properly functioning. In the event that all of the lights seem to be dim or the lights are not on at all, inspect the trailer’s light harness connection right at the truck.

Your first port of call would be to unplug the harness and then use a tester. The trailer light tester, when plugged in, would immediately inform you if the problem stems from your truck. If there is a problem then you have to check that all your truck’s lights are functioning as they should.

Inspecting would help you determine if your truck has a burnt out light, blown fuse or bad flasher. Ensure the connector is clean and then check again. Additionally, you can check the connector for any faulty or damaged wiring.

If all the lights on the tester appear to function properly, then the problem likely lies in the trailer’s harness or lights. If this is the case, we move on to the next troubleshooting step.

Clean the connector plugs

Before you begin, the socket and plug should be cleaned using a specialized cleaner. This is the step to take when you realize the lights aren’t on or appear dim. The connector plugs should be cleaned using an electrical contact cleaner and then a wire brush, preferably a fine one should be used to clean the contacts.

Thoroughly inspect the trailer’s ground wiring

Take the time out to thoroughly inspect and clean the ground wire connections. Once that is done, you should ensure that the ground wire is fastened securely to the trailer. The majority of running light issues tend to happen because of an issue to the ground wire.

Loosen the trailer’s ground screw so you can sand its wire terminal as well as the chassis contact using sandpaper. If while inspecting and you notice the screw is corroded, you have to replace it.

Replace any bad light bulbs

If the steps above have not solved the problem causing your trailer’s running lights to not function correctly, then it could be that there is an issue with the bulbs. simply, your bulbs have gone bad.

Remove the bulbs by slightly twisting and ensure the ones you install work correctly before twisting in the reverse direction. If it happens that just a single light is out in your trailer, then you might just need to change only that bulb.

This can be done by removing the lens, replacing that bulb, and then inspecting it to ensure that it works correctly. If it does work as it should, then you are set.

Clean any corroded sockets

Corrosion can be removed using sandpaper and a small dowel that enables you to get into small spaces. After doing the above steps and you still have trailer running lights that do not work, it could be that the socket and the contact points have been corroded.

To help you clean this, you will require some 220 grit sandpaper as well as a three- eights dowel and some hot glue. The sandpaper should be stuck to the dowel’s butt end using hot glue. This will allow you to reach places usually inaccessible.

The contact points can be cleaned by spinning your dowel, at the same time you move it from one side to another. Once the contacts have been cleaned, you can apply some dielectric grease to the contacts before you insert the bulb.

At this point, the bulb should be functioning properly. However, if it still doesn’t work, you might want to inspect the mounting bolts to ensure that there is adequate contact between them and the trailer’s frame. When your inspection turns up corrosion, scrub it away using sandpaper. Check your lights again.

Conduct a continuity test

The trailer’s wiring should be checked by conducting a continuity test. This is done by connecting the connector pins to a jumper wire and the sockets to a continuity tester. The continuity tester consists of a battery and a light bulb.

When the circuit is functioning properly the bulb lights up. You can utilize alligator clips to ensure that the continuity connection is easy and quick. If the running lights on a particular side still aren’t functioning, then it could be that there is a break in the wiring. 

You can search for broken wiring by searching for the wire at the connector located in front and its back end that connects to the socket. A jumper cable should be connected to a connector pin using the alligator clips and the other end should be connected to your continuity tester.

If after doing this, the light still doesn’t come on, then it is more than likely there is a break or bare spot. What you should do in this instance is to search for the break. When you do locater the break, the wire should be cut at that end and a new connection soldered. The insulation should be repaired using heat-shrinkable tubing.

The last plausible solution would be to replace the entire light system on the trailer

You should not hesitate to change the trailer’s entire wiring if it appears to be extremely corroded. When no other solution works and you keep discovering corroded parts, it is possible to purchase a wire harness for a reasonable price. With a new wire harness, you get a connector, lenses, and of course lights.

Correct trailer maintenance

Proper maintenance of any equipment you have can help you not only extend the lifespan of your equipment but it can also reduce any unexpected costs. Here are a few steps you can take to ensure that your trailer is properly maintained.

Tire Inspection

Your tires have to be inspected, particularly if the trailer wobbles. Things such as uneven tread, cracking, and bulging should be checked. You should only every check and change your tire’s pressure when they are still cold. Air should never be released or added when hot. Any tires that are damaged or worn should be replaced as soon as possible.

Cable Inspection

All the cables, as well as the cable attachments, should be inspected before you use the trailer. 

Watch where you park

You have to ensure that where ever you select to park and then set up your trailer is level and flat. If your trailer has a side awning or a slide-out, ensure that you have enough space for it. If parked near a tree, watch out for any overhanging branches that could damage the trailer if they fall. Your wheels should be blocked before you open the trailer out.

Sanitize your water system

You must sanitize your freshwater system frequently. This helps to prevent bacteria growth and odors from forming. The water filter should be replaced at least once per year. You could also change it when the flow of water begins to slow.

To do this, you have to first drain the tank, with a bucket underneath. The fittings underneath and above the cartridge should be unscrewed. The filter should then be removed and a new one put in its place securely.

Dump your tanks

If you do intend on storing your trailer for a long time, it would be beneficial to dump your grey and black water tanks. If they are not the recommended two-thirds of the way full, you can fill them up with water and then dump.

Once empty you should clean them, especially the black water tank to ensure that there is no solidification going on that can hamper the tank’s effectiveness.


It can be quite frustrating when your trailer’s running lights are not functioning as it should. More so because if you get on the road without functioning lights, you could be breaking a traffic law with the result being a massive fine.

This is why you should take time to inspect your trailer, to ensure that all the lights are working. And if you discover that your lights are out, you should try to diagnose the issue and solve it as quickly as possible.

The tips highlighted in the article can help you find out what exactly is wrong with your trailer’s running lights and what you have to do to fix it. Additionally, always take your trailer’s maintenance seriously. Doing so can ensure that things such as corrosion do not occur to a large enough scale.

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