Troubleshooting a Trailer Side Marker Light

Updated 09/15/2025 | Published 03/22/2017

Products Featured in This Question

Hook Probe Circuit Tester

(65)

$12.94

Miro-Flex Thinline LED Trailer Clearance or Side Marker Light - Sumbersible - 4 Diodes - Amber Lens

(409)

$8.94

Question:

Can it be bulbs, looks like four small on sale base. Thank you. trailer tail light built in side marker not working on one rear light

asked by: Carey

Expert Reply:

There could be a few things that might be cause your malfunctioning rear sider marker light. You will have to examine a few thighs to determine that issue. Most light today a sealed units that will have to be replaced in full instead of replacing a bulb.

First, you will need to remove the light assembly from the side of the trailer and inspect the wire connections. Some of the light will have a removable plug that connects to the back of the light housing. if you have this, then make sure the plug is securely plugged in to the housing. If you have just wires coming out of the housing, then inspect the wires for any damage that could be causing the light to not work.

Using a tester like # PTW2993 on these wires can tell you for sure if you are getting power from the vehicle back. If you a power signal is detected, then most likely the light is bad and needs to be replaced.

If the unit is sealed, we have full replacement lights, like # MCL63AB. However, these lights do come in various sizes so you will need your current light's dimensions to find a replacement.

I have attached the product page that will list all the clearance lights we carry for you to review once you have found your correct size.

expert reply by: Jared P

Les

9/14/2025

The right rear marker light on the side of the Jayco went out, tested it with the battery and it showed good reinstalled it no juice getting into it wires all look good. I used the clip wiring that clips the wires together. I don’t understand the problem.

Kate F.

9/15/2025

@Les Since the light works on a battery, the issue is in the wiring or the connection. Those clip-on connectors are often the weak link, they don’t always bite into the wires properly and can lose contact with vibration. I’d start by redoing that connection with a proper heat-shrink butt connector or just twist the wires together temporarily to test. Next, check for both power and ground at the light. Trailers usually daisy-chain the marker lights, so the problem could be between this light and the last working one. Also, make sure the ground is solid; many lights rely on the mounting screws for ground, and a bad contact there will kill the circuit. Finally, keep in mind that wires can sometimes break inside the insulation. If everything looks fine but still no power, a quick continuity test between this light and the previous one will tell you if the wire itself is the problem.