Funky idle went away after warming up. The battery was low when starting and perhaps that was the root cause. The internet has someone with a similar story.
Trying to start the truck with a dead battery will set this dtc also. As will connecting the battery with the key on.
I have an LLY duramax in my 2005 Silverado. I use this forum whenever I need to (which thankfully has not been in a very long time) I even got enough courage to change my headgaskets through the info on that site. it took 2 weeks every day after work but several years later. no problems.