Best Way To Troubleshoot/Test Curt T-Connector C56040

Updated 07/01/2026 | Published 06/29/2026

Products Featured in This Question

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

(3901)

$48.11

Hook Probe Circuit Tester

(72)

$13.14

Question:

Hello. Another question for Jessie about order 124286979, the Curt Trailer Harness for the Subaru Forrester. To try and avoid buying a wire tester and because I dont know how to use it which color wires to test, I skipped to checking amps at the 4 pin plug itself. It shows about 7-8 amps at the end when flashers and turn signals are on, but a wired trailer light set directly connected does not work! Now what? Thakn you foryour patience!!! Larry Cohen

asked by: Lawrence C

Expert Reply:

Hey Lawrence, welcome back. One thing I'd point out is that checking amps at the 4-flat doesn't really tell us if the harness is working correctly. You want to check for voltage, not current. A trailer light circuit won't produce a meaningful amperage reading unless it's powering a load, so the 7–8 amp reading is likely just the meter not being used in the right mode.

Since a known-good set of trailer lights won't work when plugged directly into the 4-flat, I'd go back and verify the basics. Double-check that the white ground wire from the Curt harness is attached to clean, bare metal, and make sure the in-line fuse is fully seated and hasn't blown. I'd also inspect the 4-flat itself for any bent, pushed-back, or corroded terminals. At this point, I still think the quickest way to narrow it down is with a simple 12V test light like part # PTW2993. All you do is poke the wires going into and out of the converter box and see if the tester lights up or not.

If the signals are present at the 4-flat but the trailer lights still don't work, then I'd start looking at the trailer lights or ground. If there are no signals at the 4-flat, then the issue is almost certainly with the harness installation or a defective converter box.

expert reply by: Jesse M

Jim B.

6/30/2026

I am hesitant to comment on this but, depending on the meter and settings, if an ammeter is connected between power and ground in the same way a voltmeter is connected, in essence a direct short to ground has been made. The good news is that should have simply blown the fuse. While a good meter can do everything you need and much more, the 12v test light is simple to use and (almost) impossible to cause any damage. You may find this a very handy device to have in your tool box. Really, I believe Jesse has given sound advice.