No start

[Reed]

2006 5.9 Cummins Auto  45,000 miles. All original. Been running fine. Went to start it up one morning and it fired right up and then the horn started blowing, I immediately turned it off as i thought i hit the panic button. Try again and nothing, won't turn over or even click the starter. All lights are working, radio etc. Batteries are showing 12.5 volts. Read something about key fob batteries, changed them, nothing changed. Pulled cables off battery terminals (4) and cleaned terminals and hoped while disconnected it would reset security feature. Replaced and then it would crank but not start. Check engine light flashing quickly and fuel pump in the tank ran constantly.  Check wiring at batteries again and found 1 little black wire on the negative terminal of the drivers side battery was loose ( i had forgot to  tighten it) tightened it up and now NO flashing engine light and NO fuel pump constantly running BUT now it is back to nothing happening when i turn the key.   Batteries still at 12.5 and i'm not sure how old they are, bought the truck a couple of months ago from the original owner and he doesn't know remember how old the batteries are. They are Mopar but surely they can't be the original batteries. My friend, the original owner is the type of guy would just take to the dealer and have them put in batteries, so they could be replacements.   I'm not sure where to go from here. Is it a security feature problem or an electrical problem or as one suggestion the key switch tumbler problem ( since i have had it, it is hit or miss if the radio comes on when i fist crank it up) so there might be something with the key tumbler??????  I thought about replacing the batteries just to be sure but didn't want to spend the money if i don't have to. 

Any help is appreciated! 

[notlimah]

Should start with getting codes. Hard to do any sorta diagnostics when it’s a lot of random things happening.

[santan40]

The thing that sticks out to me, is the horn honking, it sounds like a theft deterrent malfunction.