Got a P0621 code on my 2005, changed the alternator, no change. I'm leaning toward a BCM problem. DIC reads incorrectly, 26 volts after startup and 19 volts after running a while. Actual voltage is 14 volts after startup and then drops to 12.6 after running a while. Any opinions? The dealership is no help at all, they are the ones who said it was the alternator. I used to have access to the GM eSI which led me to believe it is the BCM but I can't open it on windows 8 to confirm.