Help! Weird Issue L99/6L80E

[ErikLS2]

This is a real thing as Jody says. When I was a mech we'd have several models that wouldn't start, but engine would turn over like a new battery. We noticed various electrical things wouldn't work right, tilt column or auto folding mirrors would stop mid travel while cranking, all while the engine was turning over just fine. We chased those for a while until deciding to try a new battery and that fixed it. It then became a quick battery diagnostic tool once we were aware.