The first thing should be to have it check for DTC'S - diagnostic trouble codes . Could have ignition switch problem , a shorted sensor bring down the class 2 data serial data network , the control modules all communicate on this data network . The starting system on this vehicle is controlled by the PCM - engine computer ! When you turn the key to start a voltage signal is sent to the PCM , if the PCM sees a voltage signal from the transmission range sensor and thier is no anti-theft system problems the PCM will energize the starter relay .
012 GM Class II Communications
There is also a direct redundant class 2 serial data circuit between BCM and PCM.
The following modules communicate on the class 2 serial data circuit:
• splice pack - SP205 attaches to:
• Air Suspension Module (ESC)
• HVAC Control Module
• splice pack - SP206 attaches to:
• Audio Amplifier
• RSA Control
• splice pack - SP207 attaches to MSM-Driver and SDM
Hooking up a scan tool an checking scan data would be a first step . Testing electrical connections , B+ voltage at the fuse boxes ,main power feeds etc. Looking at wiring diagrams an power distribution diagrams . Using a voltmeter to test with.
Electric Testing Techniques You Need to Know
Free wiring diagrams here http://www.bbbind.com/free_tsb.html
Enter vehicle info. year , make , model an engine ! Under system click power distribution then under subsystem click on power distribution . Click the search button then the blue link. Look at the power flow path an test with volt meter . Could be something simple but then again ! The complexity of the modern vehicle is well above most peoples scope .