re: Diagnose this truck electrical problem

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If the battery hasn’t been replaced, go ahead and replace that too.

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As I said earlier in the thread, even a battery that tests ok can cause problems, and his is 5-6 years old if it hasn't been replaced.

I don't care to throw parts at a problem, but this is one I would do.

Check ground connections first

also I had a lot of electrical issues in my 2015 gmc sierra. But those run from 2014-2018 so I know your 2019 will be slightly different but one thing I would say to look at, because your truck won't code it, but take off your dashboard and look towards towards the windshield glass and look around on your drivers side and start looking at the sound dampening carpet/foam shite they put in to block out the engine noise. What you'll notice, atleast for my year model is that sound dampening material will be overlapped on your ground wires and it causes an issue.

I cut that material away from the groundwire and have had 0 electrical issues since. seriously. idk how they don't have videos on youtube about this.

There's a ground contact under the backside of the passenger side front tire. You'll have to remove the tire and the felt liner to get to it. It gets corroded and causes issues. There's another one under the dash in front of the steering wheel, but that's less likely. I took the entire connector box on the top of the battery apart and went through it with contact cleaner and a wire brush, then put it back together with dialectic grease on it. Those two - cleaning the ground and the contacts - really made a difference when cranking.

The dealer will probably tell you it's the BCM (and it may be, now). I had two bad AGM batteries in a row as well. The battery ground seems to work itself loose over time, too, so check that.

The electricals on the current Chevrolet trucks are horrendous, along with a lot of other things, including squeaky seats that GM can't seem to figure out.

The wires from the body to the driver's side door are also bad to break after opening the door a few thousand times. Taking the speaker out makes accessing them a lot easier, but it's still a MFer to get to them.

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Battery is about six months old. Tested good last time it was in the shop.

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Testing good doesn't always mean much, I was amazed the first time I saw it, but since it's fairly new I'd double check that the connections are clean and tight.

Check all your positive connections at the battery also. One of my son’s trucks was acting funny, wouldn’t start and was throwing codes when it was just a bad corroded connection on the positive cable. It had three or four connections on the positive. Cleaned them up and it cleared up the starting issue.

Have a Tundra. 220,000 miles. No problems so can’t help.

If it still has the original battery, it's time for a new one.
Modern vehicles are hell on batteries.

Saw a YouTube about this. NE truck and the ground cable rotted (salted roads) from the transmission to the chassis. $20 fix. It was on a second steering module from the dealer because that's the codes it was throwing but the dealer can't get off script from the trouble shoot flow chart. Ground check wasn't part of it.