My understanding of dual alternators: The larger one charges all the time. The smaller one charges as needed, like right after start up or heavy electrical load.
As to failure, you'd think you'd get a code but I don't know.

 

They think it's a bad diode bridge in one alternator. Only way to test them is to pull them off.

 

I had many of these same gremlins with a 2017 when the notorious plug back near the spare tire had some water intrusion and corrosion. Dealer replaced it under warranty and it never acted up again.

 

I had checked that a few months back after reading about issues it was causing. I put dielectric grease on it before I put it back together. The fact that it's only doing it at idle makes sense if it's an alternator issue. I've got an ESP but I hate taking it in. I may just pull them off myself and have them checked.

 

Dealer called today, said the tech found a connector with a pin that was making a poor connection. Im not sure what connector it was. They ordered the harness though, I'll find out which one it was once I pick it up.

 

I’m curious to know if it was under the dash where the problem was found on mine for the same issues. Dealer repaired the connection on mine and no further issues. Also fixed another one same way.

 

I will follow up once I find out.

 

Got my truck back today. It ended up being the infamous grey connector on the passenger side by the hitch. I had checked this connector for water intrusion even though I applied dielectric several months ago. The connector was dry however the pins had poor connection.

 

Thanks for sharing the solution. Seen a lot of this type of issue popping up. Most seem to be on the plugs or harness in that area or a BLIS taillight. Only one I know same solution as mine and was local. I did post that solution and appreciate everyone that follows up with their solution.

 

Here is the part number for that connector.