Charging system stuck st 13.4v

[ReiSabre]

This may be dumb questioning, but I have an 08 grand prix gxp, ran fine and let it sit for a month with some on and off driving in between. At some point, charging system stopped working, and charging doesnt go above 13.4-13.5v, and won't give more when going lower from accessories or cold starts. I do have a b1516 code and current sensor has correct sensor replaced, only other things found are the ground wire to bcm from battery shows about 3-4v when reading, and not sure what the current sensor pins should read. It does charge higher and adjust if i clear the b1516 code until it comes back as well, and drops back to 13.4v. I'm still very much novice, so some insight and help would be great.

Edited March 18 by ReiSabre

[carkhz316]

So when the alternator fails to receive voltage control from the ECM, it'll default to 13.8 as a fail-safe strategy. This lower system voltage is what you are seeing as a symptom of the problem. In regards to the DTC you're getting, there are two different symptom bytes which are key for diagnosis: "66" which is you have the sensor installed backwards or "08" for signal invalid. The latter can be either from a bad sensor (just because its new doesn't mean its good) or a wiring issue. The battery sensor is a 3 wire hall effect that has a voltage reference (12V+), as well as a signal and low reference (ground) to the BCM. If you're confident the sensor is good and oriented correctly, then I would check the wiring.