Diagnosing Trailer Lights That Stopped Working
Updated 08/07/2025 | Published 07/04/2025
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Question:
On our way south and our trailer lights quit
asked by: Virgil
Expert Reply:
Sorry to hear that, Virgil.
This could be caused by a few different things and there is one way to narrow down if it's the trailer or vehicle quickly and easily. Find somewhere near you that sells trailer parts/works on trailers or someone you know in the area that has working trailer wiring on their vehicle and plug your trailer into it. If the lights work now, it's the vehicle. If the lights still don't work, it's the trailer.
If it's your vehicle, there will be a few things you will want to check. It will also depend on if this is factory wiring or aftermarket.
Factory:
-Make sure trailer plug is fully plugged into vehicle plug
-Check fuses (specifically any related to towing functions)
Aftermarket:
-Make sure trailer plug is fully plugged into vehicle plug
-Test the functions on the vehicle side trailer plug
The first one for both sounds obvious, but you would be surprised how often that's all it is. Fuses do go out every now and then, but typically it's caused by a short. Make sure the posts/pins are nice and clean as dirty one can very easily cause this. You will need some kind of tester for an aftermarket system and you will want to start at the plug and work your way back. These also go bad eventually and may need to be replaced. If you let me know what vehicle you have, I should be able to find you a replacement if needed.
If it turns out to be the trailer, it is honestly quicker and easier to just replace the wiring (depending on your trailer). A 4-flat like the # A10-4225VP is inexpensive and you won't have to try and trace the wires to find the problem, which can be a pain. If you have a 7-way, it may be better to test the lighting wires and replace as needed. Having a junction box like the # e99011 does make this process easier as well.
What kind of trailer and what vehicle do you have?
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Lazer S.
8/7/2025
Hey Virgil, How did it go with troubleshooting your trailer lights? Did you find out if it was the vehicle or the trailer causing the issue? I'm curious because your experience could really help others facing similar problems. Let me know if you need any more assistance!