P code scan will only read powertrain codes. You also have B , C and U codes that a P scanner will not read. For example U codes relate to how CAN (Computer Area Network) system is communicating with all the modules in the car including the Mylink system. There are over 25 modules in the CAN system. For example my Tahoe has 41 modules in it's system.
I'm sorry to say that you need a high end scan tool that has the ability to read all the codes to properly diagnose any electrical / electronic issues to help determine which module is affecting the system and which module is not working.
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