07 Chevy Silverado cranks over like it's going to start but dies
 

I have a 07 Chevy 1500 V6 which was stolen a few months ago. I recovered truck, but whoever stole it cut off the Cadillac converter and messed with ignition. I took off the key lock pin in ignition to get it to get it to shift into neutral, and also welded a pipe where the Cadillac converter was. Now when I try to turn over the truck it tries to start up but instantly dies after. I figure it's something electrical telling it to shut off, but I am open for suggestions. I can't afford to replace everything on truck trying to figure it out. Any suggestions???