Troubleshooting Brake Light not Working on Trailer

Updated 03/12/2018 | Published 03/10/2018

Question:

I wired a 4 way flat on a 2017 Toyota truck everything works but the break lights on the trailer

asked by: Eddie

Expert Reply:

If one one function on your trailer is not working, there are a host of potential problems you'll need to troubleshoot. In order to determine the specific problem, you'll need to use a circuit tester such as # PTW2993 to test the 4-way on your Toyota after disconnecting it from your trailer. Check the wiring going into your harness from your taillights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring. If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring.

Another issue you will want to take a look at are the ground connections, as they can be the cause of numerous lighting problems. You'll want to make sure your grounds are securely attached to bare metal surfaces free of any paint, dirt or corrosion of any kind. Also check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged. I've added links to a pair of help articles on trailer wiring that you may also find useful.

expert reply by: Kyle S