This is going to be very difficult to diagnose without the diagnostic GM software connected to the vehicle to talk to the modules and check the live data and parameters.
Here is a flow chart for a no crank/ no start:
•Ensure the Energy Storage Battery Pack is fully charged.
•Ensure the 175A Hybrid fuse in the under hood fuse block is functional.
•Always perform the Hybrid Battery Service Disconnect/Connect before diagnosing or servicing any PHT systems.
•It is imperative that the 3-phase cable connections are clean and the fasteners are torqued to specifications. Torque specifications are 14 N•m +/- 2 N•m at the starter generator, and 15 N•m +/-2 N•m at the SGCM.
1.Turn the ignition OFF.
2.Disconnect the 3-phase cable from the Stator (Electric Machine). . Test for shorts between the stator connections, and to chassis ground.
⇒If a short is confirmed, replace the Starter Generator Stator.
3.Disconnect the 3-phase cables from the SGCM. . Test the 3-phase cables circuits for shorts between cables, and short to chassis ground.
⇒If a short is confirmed, replace the 3-phase cables.
4.If no trouble found, replace the SGCM.