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Troubleshooting 4 and 5-Way Wiring Installations

Wiring issues can be frustrating and time consuming to fix, especially when you are not sure where to begin troubleshooting. When your trailer lights aren't working, your trailer is not working, and you are losing valuable time and money. Using the following information and testing procedures, you should be able to locate and eliminate the most common issues encountered during or following a 4 or 5-Way wiring harness installation and get back on the road as soon as possible.

Where to Begin

When it comes to troubleshooting a wiring installation, testing is everything. A lighting problem can originate at any point along the wiring on either the tow vehicle or the trailer, so it is important to determine not only what is causing the problem, but where it is located. The question the troubleshooter will find themselves asking is "There are so many things to check. Where do I start?"
Vehicle Wiring System versus Trailer Wiring System Image
Perhaps the most helpful thing you can do to start troubleshooting is to determine whether the problem is on the tow vehicle or on the trailer. When you use your trailer to test, you have no way to know for certain whether the problem stems from an issue with the wiring harness because the trailer's wiring system is still part of the equation. Testing the vehicle without the trailer will allow you to separate the wiring system into manageable sections.Use the testing procedure described in Method 1 to check for function at the vehicle's 4-Way plug. If all the functions come out correctly at the vehicle plug, you will know that the problem only appears when the trailer's wiring system is connected and you can concentrate your efforts on the trailer.

What Tools Do I Need?

The most helpful tool to use will be a 12V probe-style circuit tester. Click here to see our circuit testers. In addition, you will want to have basic wiring tools on hand. These include electrical tape for repairing connections, a wire stripper for cleaning up wire ends, dielectric grease, and various wiring fasteners like butt connectors, quick splice connectors, or ring terminals, depending on the type of repairs required. Wiring kits that include these items are available.A trim fastener removal tool can reduce the time needed to access your tail light wiring, particularly in SUV and minivan applications, but a flat-head (slotted) screwdriver will often work just as well. A power drill will be used to install the screws for the ground wires. A 12V battery can also be useful for testing trailer lights in situations where you don't have access to the vehicle or you suspect an issue with a particular trailer light.
Troubleshooting Wiring Tool Checklist Image

Common Wiring Issues

ProblemPossible IssuesTesting Procedures
Trailer lights work except for one function: brake, right turn, left turn, or tail.
  • Harness wires are not connected to vehicle correctly/securely.
  • (Plug-In Only) A set of connectors is not seated together properly or is not making a good connection.
  • (Plug-In Only) A required fuse or relay is blown or missing.
  • (Hard Wire Only) Vehicle has separate turn signal and brake lights, brake wire is not connected.
  • Insufficient ground or short to ground on trailer or vehicle side.
  • Method 1
  • Method 2
  • Method 3
  • Method 4
None of the trailer lights work for any functions.
  • (Powered Converter Only) 12V power wire is not connected to vehicle battery.
  • (Plug-In Only) Harness is made for models with a factory tow package, vehicle does not have tow package installed.
  • (Plug-In Only) A required fuse or relay is blown or missing.
  • Harness does not have a connection to ground.
  • Harness has been overloaded.
  • Connect 12V power wire directly to vehicle battery.
  • Use correct plug-in harness for vehicles without a factory tow package, if available.
  • Method 1
  • Method 2
  • Method 3
  • Method 5
Trailer lights worked when the harness was first installed, but eventually stopped working on one or all functions.
  • Ground location for harness or trailer connector is weak or ground connection has loosened over time.
  • Harness has been overloaded due to excessive draw from lights or a short in the trailer wiring.
  • Method 1
  • Method 3
  • Method 4
  • Method 5
Using one turn signal engages the turn signals on both sides of the trailer.
  • (Hard Wire Only) Vehicle has combined brake and turn signals, brake light wire on harness is not grounded.
  • Insufficient ground or short to ground on trailer or vehicle side.
  • Method 1
  • Method 2
  • Method 3
  • Method 4
Turning on head lights in the vehicle causes one or more of the trailer lights to stop working.
  • Insufficient ground or short to ground on trailer or vehicle side.
  • Harness is overloaded when tail lights are engaged because the trailer has too many running lights.
  • Method 1
  • Method 3
  • Method 4
  • Method 5
One or more functions stays turned on, even when the vehicle ignition is off.
  • (Hard Wire Only) Incorrect connection to vehicle wire.
  • Insufficient ground or short to ground on trailer or vehicle side.
  • Harness has been overloaded.
  • (Powered Converter Only) Trailer has LED lights which can be powered by the low-level residual voltage present at the 4-Way plug.
  • Method 1
  • Method 2
  • Method 3
  • Method 4
  • Method 5
  • LED lights may remain on dimly whenever trailer is connected to vehicle. Disconnect trailer when parked for extended periods.
Harness functions correctly at the vehicle side 4-Way plug until trailer is connected.
  • Insufficient ground or short to ground on trailer.
  • Harness is being overloaded when trailer is plugged into vehicle.
  • Method 3
  • Method 4
  • Method 5
(5-Way Only) Trailer reverse lights or reverse lockout function are not functioning.
  • Fifth wire from harness is not attached to reverse light circuit on towing vehicle.
  • Insufficient ground or short to ground on trailer.
  • Attach fifth harness wire to vehicle reverse light circuit.
  • Method 2
  • Method 3
  • Method 4

Basic Testing Procedures

Method 1: Testing for Function

At the 4-Way Plug
  • Use a 12V probe-style circuit tester to check for function at the 4-Way plug. Click here to see our circuit testers. (Figure 1). Have a helper sit in the front of the vehicle to engage the functions while you test.
    • (Powered Converter Only) Before testing the harness for function, remove the fuse on the power wire for 30 minutes, then reinstall it. This fuse should be located close to the vehicle battery, contained in a fuse holder. If the powered converter box has engaged its circuit protection feature, this will reset the box unless it was overloaded to the point where the internal connections were damaged.
    • Do not plug a trailer into the 4-Way plug until function has been verified with a circuit tester.
  • If any function does not have a correct power reading at the 4-Way plug, test the wiring going into the converter box from the vehicle side.
  • If functions are correct at the 4-Way plug, proceed to testing on the trailer.
Testing Function of 4-Way Plug Image
Behind the Converter Box
  • Test to ensure that the signals are going into the converter box from the vehicle side (Figure 2).
    • 2-Wire vehicle: Green (passenger side) and yellow (driver's side) wires should carry turn signal and brake light functions.
    • 3-Wire vehicle: Red wire should carry brake light function, turn signals should be on green and yellow wires.
  • If any function does not have a correct power reading, look for:
    • Plug-in harness:
      • Connectors that are not securely seated together or were not plugged in flush.
      • Loose wires behind connectors.
      • Missing tow package/trailer wiring fuse(s) or relay(s).
    • Hardwired harness:
      • Loose ground connection or weak ground connection.
      • Wires not connected to the correct wires on the vehicle.
Testing Function of 4-Way Plug Behind the Converter Box Image

Method 2: Verifying Correct Wire Connections

Hardwire Installation
  • Make sure that each wire is connected in the correct location (Figure 3). Standard wire colors are listed, but check connections by function if colors vary.
    • Brake wire must be grounded with white wire if vehicle has combined turn signals and brake lights
    • (Powered Converter Only) 12V Power wire should be connected directly to the vehicle's positive battery terminal with the included fuse installed in line. This wire is required for the harness to function.
    • (5-Way Only) If trailer requires a connection for back-up lights or a reverse lockout feature, ensure that the fifth harness wire (typically blue) is connected directly to the vehicle's reverse light circuit.
How to Verify Correct Wire Connections Diagram

Trailer Wiring Color Code

ConnectorFunctionHarness Wire ColorWhere to Attach - Vehicle SideWhere to Attach - Trailer Side
4-Way and 5-WayRight TurnGreenVehicle's right turn signalTrailer's right turn signal
4-Way and 5-WayLeft TurnYellowVehicle's left turn signalTrailer's left turn signal
4-Way and 5-WayGroundWhiteVehicle ground point - metal, uncoated, rustproofTrailer ground point - metal, uncoated, rustproof
4-Way and 5-WayTail/MarkerBrownVehicle's tail light circuitTrailer's tail lights
Only 5-WayReverseBlueVehicle's reverse light circuitTrailer's reverse lights or electric lockout solenoid
This chart is a typical guide; wire colors may vary based on manufacturers. Use a circuit tester to verify connections.
Plug-In Installation
  • (Powered Converter Only) Make sure the 12V power wire is connected to the vehicle battery and that the inline fuse is not blown.
  • If the harness is intended for vehicles with a factory tow package, verify that vehicle is equipped with tow package and that all required fuses/relays are installed.
    • Contact dealer with VIN to verify vehicle equipment.
  • Confirm that the part number on the harness is correct for the year, make, and model of the vehicle.
  • Ensure that harness connectors are installed on the correct sides of the vehicle.
    • Green wiring goes to the passenger side.
    • Yellow wiring goes to the driver's side.
  • Make sure connectors are seated together properly (Figure 4).
    • Disconnect the harness from the vehicle and reconnect it, ensuring connectors click together.
  • Inspect vehicle and harness connectors for:
    • Bent or loose pins.
    • Loose or damaged wires.
    • Broken locking tabs.
Verify Correct Wire Connection for Plug-In Installation Diagram

Method 3: Checking Ground Connections

On the Towing Vehicle
  • Check the ground area for any paint, corrosion, or buildup.
    • If any is present, thoroughly clean the area until bare metal is visible.
  • If using a factory ground screw, verify that no other ring terminals are stacked below the ground for the wiring harness.
    • If present, move ground for harness to a new location or move it to the bottom of the stack (Figure 5).
  • If ground area is already clean, disconnect the ground wire and attach it to a long piece of wire. Run this wire to the vehicle's negative battery terminal for testing purposes.
    • If this solves the issue, the ground wire can be permanently run to the negative battery terminal or moved to a new location with a verified connection to ground.
Checking Ground Connections on Towing Vehicle Image
On the Trailer
  • Make sure that the ground wire (usually white) on the trailer connector is securely attached to the trailer frame.
    • If the trailer tongue folds, place the ground connection on the main body of the trailer frame, behind the tongue hinge.
  • Check the ground area for any paint, corrosion, or buildup.
    • If any is present, thoroughly clean the area until bare metal is visible.
  • Move the ground wire to the trailer frame if it is attached to an aluminum section of the trailer body.
  • Each component should have its own ground for the best performance (Figure 6). This reduces the risk of a ground issue on one component impacting the whole wiring system.
Checking Ground Connections on Trailer Image

Method 4: Finding Other Possible Issues

  • Inspect the wiring for any damaged areas where copper is visible or any spots where the jacketing on the wire is broken.
    • Repair or replace any damaged wire sections (Figure 7).
    • Pay close attention to areas where wire might run near moving parts.
    • Inspect and replace any butt connectors, splice connectors or direct splices that may have worked loose or are not making a secure connection.
  • Check the trailer and vehicle plugs for corrosion.
    • Corrosion will usually be green or white in color.
    • If present, replace the plug or clean it thoroughly with battery terminal cleaner and a wire brush.
  • If the trailer lights ground through the mounting hardware, make sure that the light mounting area is clean and paint-free, and that the surface is not made of aluminum.
    • If the mounting surface is made of aluminum, connect a wire to the light stud and ground it to the trailer frame.
  • Verify that the bulb in each light is functioning correctly.
    • If damaged, replace the bulb or light assembly as needed.
Looking for Other Wiring Issues Image

Method 5: Checking for an Overload Situation

  • Check harness instructions for maximum amperage rating. Determine amperage draw of trailer lights and confirm that lights do not draw more power than the harness can handle. View the table below for amperage draws for some of the most commonly used trailer lights.
  • (Powered Converter Only) If the trailer lights are overloading the harness, it is possible that the unit can be reset if the overload is not extensive.
    • Remove the fuse from the power line and leave it out for 30 minutes before reinstalling it. Then, use a circuit tester to check for function at the 4-Way plug.
    • Do not plug in the trailer before testing the vehicle's 4-Way plug.
  • If all functions are present after resetting the unit, this indicates that the trailer may have a short that is causing an overdraw on the system, or that the trailer lights draw more amperage than the harness is rated to handle.
    • Try removing bulbs from additional clearance lights and connecting the trailer to the vehicle. If the harness continues to work correctly with the bulbs removed, this points to an overdraw from the amount of lights on the trailer.
    • Remove some clearance lights if possible or replace lights with the LED versions which draw much less power.

Amperage Draws for Common Trailer Lights

Incandescent Light

Tail Light Circuit

Stop/Turn Circuit

LED Light

Tail Light Circuit

Stop/Turn Circuit

ST70RB Part Image
.5 amps2.4 amps STL70RB
STL70RB Part Image
.054 amps.362 amps
ST9RB Part Image
1 amps2.5 amps STL9RB
STL9RB Part Image
.062 amps.259 amps
ST17RB Part Image
.9 amps2.3 amps STL17RB
STL17RB Part Image
.121 amps.279 amps
MC44RB Part Image
.2 ampsN/A MCL44RB
MCL44RB Part Image
.054 ampsN/A
MC44AB Part Image
.2 ampsN/A MCL44AB
MCL44AB Part Image
.054 ampsN/A

For more information, see:

Trailer Wiring DiagramsRouting the Power Wire for a Trailer Wiring HarnessUpdated By: Dani S.Last Updated: 3/2/2021

Frank O.


I installed the trailer wiring module on my 2024 Jeep JLU wrangler - everything worked fine for about a month - however, now the trailer lights are on all the time when I plug in my trailer into the 4 wire connector - lights stay on, even when vehicle is shut off - and brake lights and signals no longer work on the trailer - the lights just seem to be stuck on as running lights, with no other functionality and they will not turn off - any ideas on what to try (or did my module go bad)

Etrailer Expert

Mike L.


@FrankO Assuming you're speaking of a taillight converter like the Tekonsha # 118786, it sounds like your converter has shorted out. This happens when there's a short circuit in the trailer's wiring. Before replacing the converter harness, check the trailer wiring, working your way back from the trailer connector. Look for pinched wires, loose connections and wires with cracked or worn insulation. Get all that fixed before installing the new converter harness.

Frank O.


@MikeL thanks for the tip - if this is indeed caused by a short in the trailer wiring does that mean the converter has to be replaced as well (can a short damage the trailer wiring converter box installed in the vehicle) ?
Etrailer Expert

Mike L.


@FrankO From what you described, it sounds as if the converter box has shorted out due to a problem in the trailer wiring. If that's the case, you will need to replace the converter/harness. The converter is the sacrificial link in the system. It's a bummer that it got shorted out, but by doing so, it prevented the more sophisticated and very expensive vehicle wiring system from being damaged.
See All (5) Replies to Frank O. ∨



Hello, Looking for a little help. I have a small utilitytrailer. I had a mishap and cut the wires, they were between the ball and coupling and did not notice. I bought a new 5-wire, Four way flat harness lead. I then crimped wires together, color for color. All the lights come on when I put the truck parking lights on but I have no turn signals or brake lights. Any help would be appreciated. Thanks Tim

Etrailer Expert

Mike L.


@Tim I'd recommend checking the ground connection. The connector on the trailer should ground to the trailer tongue. The connection should be secure and completely free of corrosion. Check that and get back with me, if that doesn't do the trick we'll explore some other options.



Thanks for the advice Mike. I am getting ready to head out on vacation tomorrow so I will try your solution when I get back. Thanks
Etrailer Expert

Mike L.


@Tim Good deal, please let us know how it works out. Hope you have a nice vacation!



The running lights are on 100% of the time, even when the car lights are off. Brake lights and turn signals work well. There is a continuous circuit, but how to I turn it off? The previous wire harness has four wires coming out of it, and this has five (two browns). The browns are the running lights. Please advise

Etrailer Expert

Mike L.


@Chris Are you using a 4 pin or a 7-way trailer connector?

4 P.


Etrailer Expert

Mike L.


@4P Use a circuit tester like # PTW2993 on the trailer connector pin fed by the brown wire. If you show power on that pin with the vehicle headlights turned off, then there's a problem in the vehicle's wiring or if you're using an aftermarket trailer wiring harness/taillight converter, the harness/converter is faulty and should be replaced. If your trailer has a battery on it, it's possible that something is wired incorrectly, which allows power from the battery to bleed over to the taillight circuit.



So all my trailer lights are working (brake, turn signal, etc.) The problem is the passenger side brake light to my car isn’t working. I unhooked the plugs and connected my regular brake lights (taking the converter out of equation) and they work. Do you think it is a problem with the converter? Or the wiring that comes from the converter?

Etrailer Expert

John H.


@Michael My guess would be the converter. If I were you, I'd test the wires going into and coming out of the converter with a 12v circuit tester for proper function.



This might be a stupid question but i had my trailer hooked up to my truck while it was off and noticed there was 3 wires not connected and without thinking i hooked them all up together causing a spark now my interior lights are flashing like if there having trouble staying on and my left rear turning light wont turn on anymore. any ideas what kind of damage i have done ? also i think it was a white green and brown wire not fully sure it was dark thanks in advance

Etrailer Expert

Mike L.


@Mike The wires on the trailer or the vehicle? What's the year, make and model vehicle?



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