Oh OK so you also have a charging system problem.
If the voltage vary with the engine speed then it means the alternator is shot. The volatge should sit at arount 13.5V no matter the RPM.
The regulatior inside the alternator is not doing his job and i'll bet i doesn't charge your battery as well.
This is why you have to visit NAPA regularly.

So if you run your engine with a low or discharge battery then for sure you gonna have all sort of codes like you seeing.
Correct the charging system first.

 

Totally cursed... I swear to all that is holy from the Gods of Planet Chevrolet!
Tues I got gas, heard a tick, shut off engine checked oil, NO CRANK, NO START. Battery says 14V out of van, 9v in the ignition position II, ok crazy right!? Get under the van jump the starter, battery is toast because my neighbour who works at Meineke , knows what he's doing because its "What I do" drains the battery with "whatt he does" so my ex bought me a battery an hour ago. Popped it in and No click, no start. So now I know, there's spark from the pos wire to battery, but none from the noid to the starter. I talked to a master electrician today and (before I knew he was a master) explained my theory about why everything was breaking. And he agreed with my "path of least resistance" theory. So I guess eventually when I get everything that's electrical replaced or stabilized it will finally work. So now I'm going to to pull the noid. Then bench test that and the starter. I'm hoping by removing both ill see some stupid ground I didn't see before that's stupidly hanging. I'll keep y'all updated. And thank you, Pal. For answering consistently. Oh, and yes, the first thing I did was check and bench test all the fuses and relays.

 

Have you checked all of your ground wires?

 

Sorry for not replying gents. My "alternator" (heart) decided to overcharge my battery inside my own body and the whole damn ECU ie. Me! Needed to be reset. But I'm back! Ok so last I left y'all, I happened to be able to squish all the way under the Duck and found this nifty sight. Pic 1.

This is my incredibly long and stupid, ridiculously impossible to unbolt from the alt bracket, negative batt cable.

 

And the solenoid

My starter solenoid... Yeeeeek! Not video clip. Just screen shot from video

 

So believe it or not that damned battery cable was not only slathered in locktight to the alt bracket but yep 2 tiny tac welds as well. Wtaf!? Someone hated the previous owner. So I just got it out this morning at 5am. My next question is, (crosses fingers, toes and eyes) please someone tell me I don't have to replace the negative battery cable with this stupid OE. It's flipping expensive and ridiculous to replace. So, yes, they are both fried, the braided ground strap is a little, but both are covered in crappy this sludgy oil. (Yes, I fixed that problem too.) But do I seriously need to spend $76 flipping dollars to get an exact OE cable?

Good grief!. It has one main that hooks to alt bracket. A ground to frame by battery, and ANOTHER to frame by braided ground strap....

 

you don't need to spend that much

 

That starter solenoid looks like somebody started it a bunch of times by jumping it with a screwdriver.