Faulty security system causes Chevrolet Van not starting

I’ve been having trouble starting my 1999 Chevrolet 3500 Express Van. I took it to a garage and they said that it could be a faulty security system. Could that be causing the problem. Is it possible to disconnect the security system? Is there a way to get a schematic to repair the security system.

Your garage is probably just guessing. I’d take it to the dealer and have them diagnose the problem. You say you are having problems starting the van, but does it start eventually and run well when it does?
Onstar provides newer cars with systems to slow a stolen car down over time until it stops, and a newer system that shuts down the ignition system to prevent theft. It is possible that your 1999 Chevy has something similar that may be defective, but likely the car will simply refuse to start altogether, all of the time.

Do you have an after market security system or a factory system?  After market (third party) systems are well known for hard to define problems. 

Good Luck