2003 chevy blazer security light on and won't start

the security light indicate an error with the NATS communication line and you will need to diagonise it to.know what.exactly is wrong.with.the car.

I had this problem. After trying every trick in the book. The Chevy dealer said I needed a new Body Control Module. In the 2500 Silverado there is a fusebox under the hood on top of the left front fender. The fuse was marked TBC and was the closest to the fender in the back corner toward the firewall. Mine is a little 10 amp fuse. Hope it helps.