Starting truck starts then shuts down after only afew seconds?

I had this same problem & cause was an aftermarket security system that was disabled by previous owner but never removed & therefore backfed 14.5volts into ignition system over time & destroyed it.Fix is replace ignition control module,computer control module & ignition wiring harness,also I must add that reprogramming is also required for modules & both keys.Unless you know a GOOD GM mechanic who will do this on the side & get you the parts at his cost,this is a COSTLY fix at dealer $900-$1200,about 6-8hrs labor.