Trailer Running Lights Not Working? How to Test Different Trailer Lights Like Brake Lights and Turn Signals Not Working

Steps to fix trailer lights not working

1. Acquire Schematics or a Wiring Diagram

   In this case, the wiring diagram is on the cover, and we also use a wiring diagram we found online.
   
   

2. Turn Key on and Press the Brake Pedal

   Turn the key to the “ON” position. Place a pry bar against the brake pedal and seat or have an assistant hold down the brake pedal. This will put the brake lights on for testing.

3. Check the Terminals for Corrosion and an Electrical Connection

   Look inside trailer hitch’s terminals for corrosion or discolored terminals. Ground the test light and test the terminals for a connection. We did not get a connection in this example.
   
   

4. Check the Connector for Corrosion and an Electrical Connection

   Remove the electrical connector from behind the terminals and check the connector for corrosion. Test the connector for an electrical connection. We did not get a connection in this example.
   
   

5. Check the Fuses

   Get wiring diagram to understand the wiring. In our example, there is a fuse for each signal.
   
   Turn the vehicle off and check the fuses for a connection. Remove the fuse box cover underneath the hood, and look at the key on the back of the cover. Find the right fuse for the malfunctioning terminal, and test it with the test light. There should be a connection on each side, and the internals of the fuse should not be broken.
   
   In this example, we were missing the necessary fuses for these terminals. The vehicle from factory did not come with fuses and relays, so we installed new ones.
   
   Read more on how to test a fuse
   
   
   Tip: See if there are any service bulletins for your vehicle. A service bulletin for the vehicle in this example says you need to install new fuses and relays.
   
   Tip: If fuses are popping, it could be the trailer. If you keep finding yourself replacing fuses to make trailer running lights, brake, and turn signal lights work again, there could be too much of a load from wiring or connector issues.
   
   

6. After Replacing Fuses, Re-Test the Hitch’s Terminal for an Electrical Connection

   With new fuses, put the key in the “ON” position, press the brakes on again with a pry bar, and test the terminal again with a test light. If there is no connection, there may be a problem with the electrical connector or the wiring.