20 Most Recent 2011 GMC Sierra 2500HD - Page 7 Questions & Answers

Check the plug wiring again on the truck. You have a wire crossed , when you hit the lights and it only blows with the trailer, then there is a crossed wire on the truck. What seems to be happening is your sending power to the ground for the trailer. You can check the trailers ground with an ohm meter, then make sure your truck ground matches that pin. You may know this , but green rght turn, yellow is left turn, brown is parking lights, black is brake lights, and white is ground.

Hello and welcome to Fixya!
As a first course of action, I strongly suggest that you remove the positive battery connector. Let it sit for atleast 5mins. then reconnect it. You may just have system failure and reconnecting the positive connector can resolve the problem.
If the problem still persists then you consider the turn signal lever itself. It can be faulty and may need replacement.
Hope this helps and thank you for using Fixya! Have a good one!

OBD Code P0673refers to
The Engine Control Module (ECM) has detected a fault in the cylinder #3 glow plug circuit SymptomsPossible sumptoms of OBD code P0673
- Engine Light ON (or Service Engine Soon Warning Light) - Lack/Loss of Power - The engine may be harder to start - Engine hesitation CausesPossible causes of OBD code P0673
- Cylinder #3 glow plug circuit is open or shorted - Faulty cylinder #3 glow plug - Faulty glow plug module

What year, model and engine size there ?

You need an ohm meter and check to see if ther is power at the horns and lights. There is no programming that turns off the horn on the steering column. But I have one of these trucks and had the same problem with the lights and horns but it didn't happen at the same time. My lightbulbs burnt out and the actual horns under the bumper did to. Hope this helps.

Might have a bearing going bad in reared.

It could be the fuel pump, not the fuel filter.

The fuel pump is usually located inside of the gas tank.

Hope this helps!