Troubleshooting Running Lights on One Side Not Working

Question:

So I installed new LED trailer lights. All 3 functions work correctly on right side, but running lights don’t work on left side, although brake and turn signal is fine. I double checked ground connections, but don’t know where to begin diagnosing. Tow vehicle is not a problem, so maybe broken brown wire somewhere on left side of trailer. Any ideas about where to begin? Thanks.

asked by: Kevin M

Helpful Expert Reply:

It was smart to check your ground connections because in many cases that can be to blame for this type of issue, particularly with running lights, but if your grounds are securely attached to bare metal surfaces free of any paint, dirt or corrosion then you can move on. It may be a broken, crimped or otherwise damaged wire as you mentioned so you will need to thoroughly trace your wiring to ensure that is not the case.

Your trailer plug may also be a problem so check that and see if there is any corrosion, which will often be green or white. If you notice it you can clean it out with a brush and some battery terminal cleaner.

I've also added links to a pair of help articles on trailer wiring that you may also find useful.

expert reply by: Kyle S