Battery Not Recharging Light Issue?

Hi all, I have an 04 trailblazer LT 4.2l.

I was driving the other day and this battery not recharging light came on out of nowhere, battery voltage started to drop according to the dash. Made it to an oreillys had them throw their tester on it and battery came up as bad took that in to where I bought it (still under warranty) and had it replaced. Put the new battery back in and the battery not recharging light still on, had oreillys retest again and as soon as he hooked up to it it started charging everything was working perfect no recharging light on and dash reading 14v, until he disconnected his tester and battery not recharging light came back on and dropped to 12v? Any ideas as to what could be the culprit? serp belt is good yet, could this be a grounding issue?

Also the light is intermittent but on most of the time, say for instance I were to give it quite a bit of gas around 2200 rpm itll go away and I can see the batterys dash needle move up a little bit

Alternator is failing.

I'd have said alternator OR the computer, OR the wire harness related to the alternator.

The fact that connecting a scan tool to the computer makes the alternator work makes me think the main problem is the computer. If it was the alternator or wire harness, connecting the scan tool should make no difference, Somehow the scan tool is getting the computer to do the right thing.

But I've been wrong before. Maybe connecting the scan tool, and having it charge properly was total coincidence. (but I doubt it.)