308,453 miles on 5.3 2012 gmc sierra
 

So i have a 2012 gmc sierra. Motors got a lot of miles on it and now it's starting to fail. I got a bunch of codes on it like 9 different codes. I don't know if it's reading it right or if the computer is failing. When i drive the truckit Chugs and the engine light and the stabilitrac and traction control lights come on. I don't know much about vehicles but i do know a little. It was reading a po301 code which is a miss fire in cylinder one. I thought it i changed the wires and the plugs that it would fix the misfire. It didn't. I then took it in to a mechanic and that's where they got 9 different codes. Theysaid po301, po0521, 0121,po561,3031,3060,po0770, po0121 4wd autorange indicator control circuit short to ground or open. I am surprised becauseit send like all these codes hit at once. It was driving fine except for my voltage meter kept going down to 9 volts then back up to 14 volts then back down? I still don't know why that was doing that. I figured it was a grounding issue so i installed another grounding strap to the engine block just below the alternator. I could really use some advice. Thanks.