Ok diagnosing this problem is not going to be easy without the access of a scan tool that can actually read body failure codes such as the theft system. These tools are the $3000.00 units that the pros use. I can tell you that gm in general has a real problem with the passlock sensor. This is a sensor located in the ignition cylinder housing that actually tells the system that the key cylinder was turned and uses a coded message to allow for a start everytime you start the vehicle. There is a chance that a simple relearn procedure will repair this problem for you. Here is how to do that.
Important: Do not remove the key from the ignition during this procedure. If the key is removed, the procedure will have to be performed from the beginning.
Important: The vehicle learns the PasslockTM Sensor Data Code and/or password XXXXX the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.