Troubleshooting Curt T-Connector C56040 That Doesn't Seem To Be Working

Updated 06/24/2026 | Published 06/21/2026

Products Featured in This Question

Curt T-Connector Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

(3779)

$40.26

Hook Probe Circuit Tester

(67)

$14.32

Question:

Hi! I have the plug and play trailer harness order 124286979. See my 3.24.26 comment about finding the factory plug in my Subaru. I was very slow to finally install ant try out with some wireless 7 pin trailer lights I use for my RV. I used a 4 pin to 7 pin adapter and pluged it into the Curt unit, and they didnt work. I then tried the Curt unit with some 4 pin lights on a small trailer no adapter in the line and they dont work either. Help? Larry Cohen

asked by: Lawrence C

Expert Reply:

Hey Lawrence, thanks for reaching out. I'd want to start here by testing the 4-flat on your Subaru. First make sure the in-line fuse is fully inserted, then make sure the white wire is grounded to good clean metal free of any corrosion or paint. Also make sure the white plug is fully seated and connected to the vehicles plug.

If all of that checks out, the next step is to test the wires going into and out of the converter box. The best tool for this is a wire tester like part # PTW2993. First test the wires going into the converter box and make sure all the brake light, tail light and turn signals are normal. Then test the wires coming out of the box and make sure those signals are normal.

If the wires going into the converter are not showing proper signal, that means the harness is either not installed correctly, or there is something wrong with the vehicles harness.

If the input wires are reading normally, but the output wires aren't, this means the converter box is defective and will need to be replaced.

Check that out and let me know your findings. I'm happy to help troubleshoot further if needed.

expert reply by: Jesse M

Curt T-Connector Vehicle Wiring Harness with 4-Pole Flat Trailer Connector
(click to enlarge)

William S.

6/23/2026

I purchased the 4 pin harness for my 2023 Subaru Ascent in addition to the trailer hitch receiver. I’m fairly handy and though I never did this type of job before it was a cinch to remove the bumper and install the trailer hitch and then run the wiring using the videos from trailers website. It worked perfectly first time every time.

Jesse M.

6/24/2026

@WilliamS Awesome! Glad to hear the wiring harness is working out, and that the install didn't give you too much trouble. Thanks for sharing!

Steve G.

6/24/2026

The newer led lights require a good ground , file grind sand to bare metal to ground the system. Four to seven may not connect the same on your Subaru as there is provision for brake wiring involved. The test lights that plug in are a valuable tool to throw in your tool kit and inexpensive.