Symptoms
- Left or right side taillight/turn signal doesn't work while the other side is fine
- Brake light out on just one side
- Issue seems to follow a specific bulb location
Likely causes
- CommonBurned-out bulb on the affected sideA single bad bulb is the simplest and most common explanation for a one-sided light failure.
- CommonCorroded or loose ground connection on that side's light fixtureMany trailers ground each light fixture locally, so corrosion at just one fixture can affect only that side.
- PossibleDamaged wire specific to that side's runWiring runs to each side separately in many trailers, so damage can be isolated to one side.
- PossibleBad connection at a splice or connector specific to that sideTrailers with add-on wiring or repairs may have a splice unique to one side's circuit.
What to do
- Check the bulb on the affected side first, since this is the quickest and most common fix.
- If the bulb is fine, check the ground wire and connection point at that specific light fixture for corrosion or looseness.
- Use a test light to check for power at the socket on the affected side; no power suggests a wiring or connector issue upstream.
- Trace the wiring on the affected side back toward the connector, checking for any visible damage, especially near hinges, fenders, or pinch points.
- Check for a possible mix-up if the trailer wiring was recently repaired or modified, since it's easy to accidentally swap wires.
- Once found, repair the specific break, connection, or bulb rather than replacing the entire harness unless multiple issues are found.
Tools and parts
- Test light or multimeter
- Wire strippers/crimpers
Replacement trailer light bulb or fixtureSearch: “trailer light bulb or trailer tail light assembly”
When to call a pro
If you can't isolate the fault after checking the bulb, ground, and visible wiring, a trailer shop can trace the specific circuit.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.