Troubleshooting Side Marker Lights Not Working With Curt 4-Way

Updated 11/03/2026 | Published 10/18/2026

Question:

I have this C56218 on my 2020 Transit Connect. I just bought a new trailer and the side marker lamps are not working. The dealer confirmed the trailer lights are all good while I watched. The tail lights, brake lights, signal, and hazard lights all work. There is an issue with the tail lights on the trailer staying lit, even with the vehicle lights off. I checked voltage on the brown wire. The voltage going into the converter is 12 volts, but coming out is only 7.

asked by: Kurt R

Expert Reply:

Hey Kurt! I would start by double-checking the ground connection on your Curt Wiring Harness # C56218 and making sure it's still securely attached to a clean, bare metal surface. If your trailer has LED lights, the lights staying on with the vehicle turned off may just be residual voltage, so I wouldn't be too concerned about that yet.

The 7V reading is what I'd focus on. I would test the running light circuit at the 4-way with the trailer disconnected, then check the voltage directly at the side marker lights with the trailer connected. This will help narrow down whether the voltage is dropping at the converter or somewhere in the trailer wiring.

Are the lights on your new trailer LED or incandescent?

expert reply by: Robin H

Bill T.

10/21/2026

I would double check your connections, especially the grounds. The explanation from Robin H sound right on.

Robin H.

10/22/2026

@BillT thanks for backing me up on this! Ground connections can cause some strange lighting issues, so they're always worth checking first.

Kurt R.

11/2/2026

The ground wire is good. It is grounded to the vehicle body. I checked the voltage at the side marker lights on the trailer and the voltage is the same as at the connector, 7 volts.

Robin H.

11/3/2026

@KurtR thanks for checking! Since the trailer lights have already been tested and the voltage is the same at the connector and marker lights, I'm leaning toward the Curt converter being the problem. I'd check the voltage at the 4-way with the trailer disconnected and the running lights turned on. If you're still getting 7V with 12V going into the converter, it's likely failing. The # C56218 has a 1-year limited warranty, so since you purchased it in January, I'd contact our customer service team or Curt directly to see about getting it replaced under warranty.

Kurt R.

11/3/2026

@RobinH one of the first things I did was to make sure I had good voltage going in. The converter is getting 12 volts in, but putting out 7 volts on the brown - that's with the trailer disconnected.

Kurt R.

11/2/2026

the trailer lights are all LED