Troubleshooting Left Side Trailer Running Lights Not Working

Updated 10/03/2019 | Published 10/02/2019

Question:

Trailer Running Lights Do Not Work but Brake Lights and Turn Signals Do Work Question: Blinkers and break lights work but tail/running lights dont. Truck tail lights are working. Is it a fuse or is it wiring? asked by: Joan C SAME QUESTIONS BUT right side wrongs work, but not left side only the running lights - break and turn works

asked by: Bob L

Expert Reply:

If just the left side trailer running lights aren't working then the issue is almost certainly on the trailer side, as the running light circuit isn't side-specific on the tow vehicle end (if one side is working on the trailer you know you're getting appropriate power from the tow vehicle).

So you can start at the trailer side connector and just visually inspect it for any corrosion or debris that might be built up inside the pins, cleaning out if needed. It could be something as simple as this that's preventing a good connection with the tow vehicle.

If the connector is fine try and trace the left side running lights wire as far as you can back towards the tail light. Check for any damaged, pinched, or exposed portions of wire that need to be repaired/replaced. Also make sure the tail light itself is securely grounded to a clean, bare metal surface that's free of any rust or debris. A weak ground on the light could allow for enough power to illuminate the brake/turn signals but not enough for the running lights, which put a higher load onto the circuit.

If you still aren't able to locate and resolve the issue please let me know and we can keep troubleshooting.

expert reply by: Chris R