Trouble Shooting the Trailer Running Lights Not Working: Check Step by Step

Trouble Shooting the Trailer Running Lights Not Working

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Always keep your tow vehicle and trailer in tip-top condition to avoid unforeseeable havoc that can affect your assets and your life. Usually, the trailer running lights not working correctly is commonly encountered, but they can be fixed easily with a few cautious checks and applying their respective solutions.

Trailer running lights not working can be caused due to many reasons, like bad fuses, missing relays, bad wires, rusting surfaces, and lack of proper grounding.

Still, all these problems can be easily determined by a few checks to keep your trailer running, lights lit up, and carrying a safe journey on the road.

Be always prepared to nip it in the bud so these problems don’t cause significant adversity.

Trailer Running Lights Not Working

Trailer lights are essential indicators on the road while driving; these lights get all their signals and power from the tow vehicle.

The trailer lights get signals for left and right indicators, for brakes, and remain lit when the tow vehicle is moving.

If the trailer lights are not working in your trailer, then the problem might be at two prominent places; first, it can be the tow vehicle, and second the trailer itself.

Trailer Running Lights Not Working

The tow vehicle and the trailer are then checked for several areas where the problem can exist.

The problem might be as petty as having paint or corrosion on the metal surface hampering the current or as big as having faulty fuses or wires.

So let’s understand the causes and their cures step by step.

Determining the Origin of the Problem

When you encounter the faulty light on the trailer, the first thing to probe is where the problem lies; you are likely to determine the origin of the problem, whether it’s with the tow vehicle or the trailer.

The tow vehicle is the primary and sole power supplier in this relationship, so we will start with the tow truck and try to rule out the possibility of any fault. There are many areas to be considered, but the first one is the electric circuit connector. Later the fuses, relays, and wiring are to be checked properly.

After ruling out all the possibilities of any fault in the tow vehicle, the attention is shifted to the trailer itself. The trailer electricity circuit has much to be checked for regarding wires, grounding, bulbs, and connectors.

Check the Tow-Vehicle Electrical Circuit

The two-vehicle is checked for proper grounding and an active and working electric circuit. The easiest way to check the tow vehicle’s electrical circuit is through the four-prong electrical adapters, which are readily available on the market.

This adapter has a simple mechanism that consists of three lights to indicate the working of the electrical circuit.

The four-prong electrical adapter is to be plugged into the four-prong plug in the tow vehicle and checked for the lights on the adaptor.

The first light is for the running light, the second is for the left and right indicators, which will be lit only if the indicators are on, and the third one indicates the brake lights, which will turn on in case the brakes are applied.

If all the lights are lit up, the tow vehicle’s electric circuit works correctly. Still, if the second and third light does not light up on the application of indicators and brakes, respectively, then there must be some problem within the tow vehicle circuit. To figure out the issue, the following solutions are to be adopted.


Suppose your truck or tow vehicle has a different plug for the trailer or RV. You can test it with the tester, attaching the grounding wire clip for the ground to the connecting point and touching the different tooth/pin in the connecting plug with the electric tester in the tow vehicle.

There is always a diagram on the plastic lid of the plug to show the function of different teeth/pins, and if the link is working perfectly, the tester will lit-up on touching the tooth in the connector. But if it does not light up, we must undertake some checks and solutions for them.

Quick Solution: Get Rid of Rust, Dust, and Debris

There is a good chance that debris and rubbish can approach the connector plug due to continuous traveling and if the lid of the plug is left open.

The plug and the prong should be brushed appropriately to eliminate debris or dust hampering the electric circuit.

There is another possibility of the appearance of rusk on the exposed metal surfaces in the connectors or the plug.

Many reasons are at work when the connectors become corroded, either due to rainwater or moisture exposure; however, they can be made clean with sandpaper so that the metal surface is bare enough for running the circuit.

Another way to clean the connector is through pressurized or compressed air into the plug to remove debris.

After clearing the metal and plug from rust and dust, the four-prong plug will show all the lights on, indicating the proper working of the tow vehicle electrical circuit.

Quick Solution: Check for Bad wires

Another problem with the prong adaptor not lighting up might be the bad wires. Continuous exposure to moisture and dust makes the wires in the truck wear and tear; this causes a break in the electric circuit and hinders the current flow.

Check the copper in the wires for rust, any cracks in the wire that might have broken the current flow, or wires in lousy shape; all should be replaced either in the tow vehicle or the trailer connector. Clean and clutter-free wires are an assurance of keeping up a fair electric circuit that will perform at its best.  

  • Wire Color and Function

The trailer running lights might have LEDs or bulbs, but the wiring system is the same; a four-wire system connects the tow vehicle with the trailer.

The green wire indicates the right signal, the yellow wire will be lit-up for the left signal, the brown wire stands for the parking lights, and the white wire is for the ground. The green and yellow collectively perform the function of brake light as well.

new trailer

In the case of crimp connections in the wiring, at times, it loosens up and makes the circuit inefficient for the flow of current, and as a result, the light doesn’t lit-up. All these discrepancies should be immediately replaced by placing new wires to ensure a proper current flow.

Quick Solution: Check the Fuses

After ruling out the problem of rust in the connectors and bad wires in the trailer and tow vehicle, the next step would be to check for the fuses of the tow vehicle.

A hood fuse panel is on the driver’s side of the car bonnet, with details about the fuses and relays on the inner side of the fuse panel lid.

There is a detailed list of all the fuses and their respective functions so that you can find them easily on the fuse panel and fix it.

Taking a 12 Volt tester, we can check whether the fuse is working correctly or blown. If the light on the tester goes on, the fuse is good to go, and if not, it is to be replaced.

The owner’s manual of the car also gives a detailed description of the placement and value of these fuses, which should also be considered meticulously for installing new fuses.  

  • Trailer Power Fuse

The trailer power fuse should be placed appropriately for the circuit to work properly. These micro fuse plugs can be checked for value from the car manual and adequately installed. If you find a blown a fuse, it is to be replaced for the circuit to start working again.

  • Trailer parking Fuse

A parking fuse and other fuses connect the circuit with the value of 3Amp or 10Amp. These micro fuses are to be checked according to the specifications on the box and fixed in place to make the running light work.

Trailer Tow Electronic Break

Another fuse must be checked for the trailer’s electronic brake indicators. It is usually 30Amp; if it must be replaced immediately with a new one.

Another Quick Solution: Check for Relays

In addition to the fuses, some relays are installed in the circuit to work the trailer lights properly. After checking the fuses, the relays should be considered for their proper working and positioning, and if they go bad, consider changing them also.

Repeatedly Blowing Fuses

Another situation witnessed a lot in the tow vehicle is repeatedly blowing fuses. This means that the power fuse or any other fuse controlling the trailer circuit might have gone amiss, and that’s why the trailer running lights aren’t working properly.

home trailer

This is usually caused by little short circuiting within the wires, if the wires come between two heavy objects and get wear and tear, that will cause the fuse to blow, and for this, the whole wiring is to be checked religiously to pinpoint the cause of the problem.

Check Grounding for the Trailer Connection

After ensuring that the tow vehicle is working properly at the connecting prong harness and the fuse and relays are properly installed, the next thing to be investigated is the grounding for the trailer connection.

The grounding on the trailer connection should be first looked at for any paint or rust that might have accumulated on it with time. It should be removed with the help of hard metal or even sandpaper to fully expose an excellent solid ground metal for the proper flow of current.

Tightening the grounding wire

When the rusk and other muck are removed perfectly from the ground, ensure that the wire end is clean and free of rust.

This area likely gives rise to the problem, usually in the current flow, and cleaning would make a difference.

Now it’s time to tighten the wire onto the ground with the help of a screw so it touches the bare ground. The tightening should be double-checked to eradicate the chance of any loose wires.

Check: How to Hang Things on Trailer Walls (Step By Step)

Check Grounding for Trailer Lights

The next thing to be checked is the proper grounding connection of the trailer lights. Usually, different types of wiring are in the trailers; the simplest one is where the grounding wire is attached to the ground spot behind the trailer lights.

The grounding wire should be attached to the bare metal to ensure the current reaches the trailer light.

The metal hole for the ground should be checked for corrosion and cleaned immediately with the help of san paper; the screws at the back of the lights should be cleaned so that the circuit is completed properly.

There is another type of light available with the trailers, these lights don’t have a ground, but the bolts at the back also act as the ground.

So when you tighten the bolts with the trailer wire, it will ground the lights themselves. These bolts should also be kept clean and rust-free for the proper functioning of the trailer running lights.

Check for Bad Bulbs

Another reason that might have made the running lights not work appropriately is the bulbs themselves might have gone bad.

The bulbs installed in the trailer should also be checked for their functioning and, if found bad, can be easily replaced with new ones by simply rotating and clicking back in at their place.

  • Bulb Sockets

The bulb sockets should also be cross-checked for corrosion or any sediments of mud or dust that might be the reason behind the running lights not working properly. These sockets can be cleaned with sandpaper and are ready to perform well.

  • Check for bad wiring.

The wires connecting the bulbs can also be in bad shape and condition; after checking the state of the wires, the faulty wires should be removed to make the current flow properly.

Checking to the ground of Trailer Lights with Tow Vehicle’s Frame

When all the grounding is done perfectly, the bulbs are checked, and the bad wires are removed; if the running lights are still not working on your trailer, there is another way to double-check for the trouble-causing area.

Take the grounding wire from the trailer lights back to the tow vehicle’s frame, and if the light works fine, then you will be sure that the grounding problem lies in the trailer. There are still some problems with the grounding, and they need to be rechecked.

heavyduty trailer

The Final Check

Once the grounding is done and all the surfaces are cleaned for a good connection between the wires, the experts advise another last test for grounding.

You can do it by shaking the trailer; if the lights flicker, then there are still some loose ends on your ground nuts, and you need to tighten them up until the lights on the trailer stop flickering if shaken. 

Other Odd Situations

The issues causing the running lights not to work might be due to some unusual situations, like in case you are using a foldable trailer, the whole trailer body is to be appropriately stretched as at times it causes a break in the grounding and hinders a proper flow of current.  

Composite or Home-made Trailers

If the trailer is made up of materials other than metal, it might create a problem with the lights’ grounding.

Metal is best for grounding the trailer; however, there are lights with bolts that act as a ground, but usually, they get rusted and hamper the current flow.

This factor should be considered while making a trailer. The grounding material should be metal and not any other alloy or non-conductive material as it will obstruct the electrical circuit, and the lights on the trailer won’t work properly.


The trailer running lights not working can create a hazardous situation and become an impeachment of law that can put you in serious trouble.

Some immediate steps that can be taken are discussed at length in this article, including checking the electric circuit of the plug, inspecting the fuses, and relays and their functioning, examining the bad wires, refining surfaces for grounding, and some more options that should be incorporated to make the running light back to work.

The issue can be resolved by ruling out each cause and moving to the next to underline the actual cause for the running lights not working on your trailer.

As we know, a stitch in time will save the nine, so it’s always better to be cautious and fix the problem rather getting into a big one.

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