I have recently purchased a gmc 2013 duramax has about 25k miles, had it about two weeks. I recently hooked a trailer to it all lights worked great. Then my left blinker quit working front to back including the dash indicator. I checked fuses all was ok. I unhooked trailer and still no blinker. After about 200 miles my blinker started working. I took the truck to gmc service they replaced multi function switch. I hooked back to my trailer and everything worked fine until my left blinker quit again. I have pulled this trailer with other trucks no problem. So now I don't know if the problem is the truck or trailer or what could be the problem any help would be great!

Also it has worked after being hooked to trailer for awhile and then quit again.

I would verify the electrical load of the trailer for each circuit. Some electronic multi-function switches can only handle so much.

sounds like you have an intermittent short on the trailer. sometimes they do that if the seals on the lights start going bad. sometimes a wire chafes to ground with vibration so it only grounds out if it touches a certain way

So trailer is not pulling much all lights are LEDs. Any Idea on witch wire could have the short causing the left blinker to quit? Also truck blinker started working again with out trailer hooked up then quit a few miles later. I feel like the problem is in the truck somewhere but I don't know. What could cause the truck bliInker not to work if the trailer is not connected? Also I really appreciate the help!!

Is your trailer wiring hooked up to the factory trailer wiring. If it came with it from the factory then yes. The factory wiring is fused seperatley from the truck wiring. I just don't know why the fuse hasn't blown if it is shorting out.

Make sure the grounds are in good and clean condition. Todays trucks have modules that control everything. If the resistance is above a certain threshold, problems will occur. Top two problems on trailers, the ground circuit and loose connections with the scotch lock blue connectors. Heat shrink crimp connectors are the best way to go.

x2 on the grounds and those blue crimp things.

i have had issues with newer vehicles having loose fuse sockets, if the feed to the blinker is intermittent it would cause you intermittent problems too.