Trailer Marker Lights Not Working — Where to Check First
You walk around the trailer doing a pre-trip check and notice the marker lights — the small lights along the sides and rear that stay on whenever the running lights are on — aren’t lighting up. Before assuming it’s a trailer wiring problem or a bulb-by-bulb issue, there’s a tractor-side check that catches a surprising number of these cases.
Check the Fuse Box Under the Hood First
On Volvo, Freightliner, and Kenworth trucks, there’s a fuse box under the hood that includes fuses dedicated to trailer lighting circuits — separate from the cab’s interior fuse panel. If a marker light circuit fuse here has blown, every marker light on the trailer fed by that circuit goes dark at once, which can look like a trailer-side wiring fault but is actually a simple fuse on the tractor.
This is worth checking first specifically because it explains the pattern that confuses people: if it were a single bad bulb, you’d expect one light out, not all of them. If it were a broken wire on the trailer, you might expect lights on one side or one section to be out. But when an entire circuit’s worth of marker lights — sometimes all of them — go out simultaneously, a blown fuse on the tractor is one of the most likely explanations and the fastest to check.
Why These Fuses Blow
Trailer connections see a lot of exposure — moisture, road salt, and the wear of being plugged and unplugged constantly. A short somewhere in the trailer’s wiring (a chafed wire rubbing against the frame, a connector with water intrusion causing a short between pins) can draw enough current to blow the corresponding fuse on the tractor side. If you replace the fuse and it blows again quickly, that points toward an actual short in the trailer wiring rather than just a fuse that aged out.
If the Fuse Is Fine
If the fuse checks out and marker lights are still dark, the next step is the trailer connector itself — the seven-way (or similar) connector between tractor and trailer. Corrosion or a bent pin in this connector on the specific circuit for marker/running lights will produce the same “all marker lights out” symptom even with a good fuse, since the signal simply isn’t making it to the trailer at all.
From there, if the connector and fuse both check out, you’re into trailer-side wiring — checking the main harness running along the trailer frame for damage, particularly at points where it’s exposed (near the landing gear, around the rear frame where it’s most vulnerable to road debris).
What to Do
Start under the hood, not under the trailer. Check the trailer light fuses in the tractor’s under-hood fuse box first — it takes a minute and rules out (or finds) one of the most common causes before you spend time crawling around the trailer itself. If the fuse is good, move to the trailer connector next, then the trailer wiring harness if needed.