Why do I not get ANY codes?

"All it will do is click when I turn the key." -- would that be a relay somewhere or the starter clicking? If it's the starter, it's not getting enough voltage. Charge the battery and jump start it. -- Try some starting fluid when you get the engine to spin. That will let you know if you should go after spark or fuel.

I turned my 2006 Ram with auto trans and Cummins off when I got home. The next morning only clicking. I have a charger so I did an overnight charge. Next morning still only clicking. Put the battery charger back on all day and night on automatic charge. Next morning still dead. Took my batteries to Interstate Battery. They said they were fine and should start. I had 1/3rd warranty on the batteries so I got new ones at 2/3rds cost. Put them in and it started right up. Don't ask why but it has worked well ever since. I thought maybe a relay or starter but started with the batteries as they cost less than everything else.