As mentioned, if the power goes out or gets interrupted unexpectedly (as seen by the vehicle) it will leave an idiot light on the dash. It's unfortunate that it doesn't give you any information about the diagnostic code, but I've become familiar because of how the crummy OpConnect EVSE at work operates.
If you want more information and don't want to be at the mercy of OnStar in the future, this is a very handy device:
http://www.amazon.com/LELink-Bluetooth-Energy-OBD-II-Diagnostic/dp/B00QJRYMFC
I've used a couple of apps with it (on my iPhone 6), and OBD Fusion seems to be the best for checking codes so far.
Bryce