Trailer Running Lights Not Working On 4 Pin: Best Guide

A 4 pin connector is used in many applications, including a utility trailer or boat trailer. Even with just a few wires, sometimes things go wrong, and you may not have things work, such as your running lights.

This post will review the common issues and the solution to fix this problem, including:

  • Ground
  • Fuse
  • Wiring
  • Connector
  • Light Bulb

Unlike a 7 pin connector, the 4 pin connector has only 3 wires and one ground.  We will focus on the special characteristics of this connector and determine why the running lights are not working.

Table of Contents


The most common issue for trailer wiring issues is the ground wire.  The ground wire is the white wire usually attached to the trailer frame by a screw. 

This ground wire is exposed to the elements, is known to corrode, and does not have a good metal-to-metal connection.  Any corrosion separating the wire from the frame will cause the ground not to work, as well as your trailer lights.

When you look at a 4 pin wiring diagram, you will see 3 pins for the lights and 1 pin for the ground.  This 1 pin is the common ground.  All trailer lights and pins use this one connection as their ground source.


Your ground wire is connected to various parts of your trailer framing. By doing this, the entire trailer frame acts as ground.  This makes it easy for a light to be connected to a nearby piece of the trailer frame to get ground.

If only your running lights are not working, the common cause is usually the ground wire going from that trailer light to the frame.  Seeing that the rest of the lights are working, your trailer is getting some ground, just not at the running lights.

Visually inspect the white wire that goes from your running lights.  Check it for any corrosion or rodent damage. If you see corrosion, unscrew the wire from the frame and clean it with sandpaper or a wire brush.  Get to shiny metal for a good connection. 

Also, reconnect with a wire nut if you see any damage or break in the wire.  Being this is a ground wire; you don’t need to be as worried about waterproofing the wire itself.

For more in-depth testing on this, visit my post on how to check your trailer ground wiring.


Another common cause of your trailer running lights not working is a blown fuse.

Your trailer lights may have a separate fuse, which can be an inline fuse.  This type of fuse will have a set amp reading that will blow a fuse for safety if there is a short that would cause damage to the lights. The inline fuse type should be waterproof, but sometimes water does get in them and can cause a short, creating a blown fuse.

Another setup besides the inline fuse is a fuse box or fuse panel.  This fuse panel will house your fuses, including your running lights.  A fuse box usually has a fuse cover that lets you know which fuse goes to where.


The next step is to check if you have blown a fuse.  Whether you have an inline fuse or a fuse box, this process is the same to determine if the fuse is bad.

Step 1: visually inspect the fuse to see if it is blown. A fuse is a plug with a wire with a certain amp rating.  Once that amp rating is exceeded, the wire inside the fuse will break, causing an instant disconnection to the light.  This will protect the light from further damage caused by excessive power.

Step 2: Use a test light.  A visual inspection is good, but a multimeter or test light is the best way to confirm. We reviewed that the trailer frame is ground, so go ahead and connect the negative clip from your test light to the trailer frame. Make sure you are connected to somewhere with bare metal. 

Next, take your test light probe and back probe each fuse pin.  You will need to probe BOTH pins, one at a time.  The reason is that you could have power on one end of the wire and not the other end.  So by testing both probes in one fuse, you know power is flowing from one end of the wire to the next.


The next step to check the running lights is the power wire.  The brown wire goes from your trailer harness at the trailer plug to the running light.  This brown wire also serves as the power for your side lights.

Brown wire is for your side lights

Note that your turn signal and brake lights use a different wire.  The wire used for those lights is the green wire. So since your signal light is still working, there is no need to look at the green wire, and let’s focus our attention on the brown wire instead.


Inspect the brown wire from the trailer plug and down the wiring harness to the light.  Similar to the ground wire, inspect for damage.  In this case, since this is the power supply of the wire, there are a couple more concerns to inspect.

  • Inspect for any exposed metal from the wire. Since this is a power wire, not the ground, you should not have any exposed wire showing.  Any exposed wire near the trailer frame (which is ground or negative) can cause a short. Putting a power wire next to the frame is just like putting the positive and negative wires together on a battery,  It will cause a short.
  • Inspect for waterproofing.  Unlike your ground wire, you want a water-tight connection with your power wires.  This is because water can enter the wire and cause a short.  This will blow a fuse and cause the trailer lights not to work.


Another potential issue to check is the connector itself.  A trailer plug is designed to fit snugly to each other to allow good metal connections.  If there is any play in the plug itself, the trailer connection will fail, and you may have lights that don’t work.


Inspect the trailer connector. Make sure the plastic housing is not damaged or cracked.  Even a tiny crack in the housing could cause a bad connection.

Check the pins on the plug.  See if any are damaged, bent or corroded. The pins should be round and shiny metal.  For more information, visit my post on how to clean your trailer plug.

Press connector tight

Also, this would be a good time to ensure the 4 pin connector is firmly connected to the tow vehicle.  The goal is to push the connectors together enough that the plastic on the truck plug and the plastic on the trailer plug meet.  If metal is still showing, the connection is incomplete, and you should push it together better.

Light Bulb

The last potential cause to check is the light bulb.  If both tail lights are out, you may not suspect the bulb.  But there are a couple of instances where both bulbs can burn out together.

Besides the age of the bulb, the most common cause of failure is water.  Your tail light housing should be waterproof.  But what happens with age and UV exposure is that the seal gets damaged, and water intrudes into your light housing.  This water can get to your light bulbs and cause a short.  This short will either blow a fuse or damage the bulb.


Check your light bulb and light housing for any signs of water.  If you see any signs of water, see if you can pinpoint where it is coming from and try to seal it.

Another good practice is to use dielectric grease at your light bulb socket. This clear grease will keep out any moisture or water that finds its way in.  Put on about a pea size amount and smear around the bulb housing.

Final Thoughts

Trailer running lights not working can be a safety concern and should be addressed soon.  The most common cause for these lights not working is the ground.  Being exposed to the elements, it is susceptible to corrosion and losing its connection to the trailer frame. Other possible causes are fuses, the power wiring, the connector or the light bulb.