I had a 'Check Engine' light show up this past week. I don't have a code to share but I'm pretty sure that I know what the problem was. I did call OnStar and they said it was a charging system failure. Today is 3/18/2016 and I've had the car, a 2012, since the end of 7/2015. It was due for 37,5K service which was just a tire rotation but I asked them to change the oil for me anyway. I decided I wanted to run the engine a bit to get the oil 'all up in it'. Also the car has had a minor oil burning smell every time the engine has run so I wanted to see if that was now gone. I usually leave the charging set to have it fully charged by 7am but this time I wanted it to not charge so that the engine would come on. When I put the car in the garage and shut it down I said 'OK' to my usual 'charge by 7' setting but then changed my mind, brought up the appropriate screen and touched either 'change charge mode' or 'temporary overide', got out of the car and plugged it in. If I remember correctly I then unplugged it from the car, unplugged the 3-prong plug from the wall and re-plugged it into a different outlet then plugged the charger back into the car. I didn't have a good reason to do that I was just being 'anal'. I left it hanging, so to speak. So it seems that I confused the computer and the next morning there was no blinking light on top of the dash, the car had not charged-that is what I wanted-but now when I started the car the CEL was on. It stayed on for 4 days...3 overnight charges...and finally went off after the 2nd time I started it following the 3rd charge.
So...I guess that like any computer, if you confuse it, it will want to be rebooted. And thankfully this one is able to reboot itself. Oh, and yes, the oil smell is now gone!