Anti-theft light lit on the instrument cluster ? This is a complex computer controlled circuit ,starting . The ignition switch is a input to the BCM - body control module and when it see's start request signal , it will energize the run crank relay powering up the vehicle . And also send's a message via the GMLAN serial data bus network to the ECM - engine control module , which will energize the starter relay turnning over the motor . GMLAN is general motors own version of C.A.N. - controller area network How to Diagnose Controller Area Network CAN
The first place to start diagnosing this problem , check battery , cable connections , check for DTC's diagnostic trouble codes !
When the ignition switch is placed in the Start position, a discrete signal is supplied to the body control module (BCM) notifying it that the ignition is in the Start position. The BCM then sends a message to the engine control module (ECM) notifying it that CRANK has been requested. The ECM verifies that the transmission is in Park or Neutral. If it is, the ECM then supplies 12 volts to the control circuit of the START relay. When this occurs, battery positive voltage is supplied through the switch side of the START relay to the S terminal of the starter solenoid.