My Check Engine light came on yesterday. About the same time, I noticed that the car was very noisy. There was a fan running at high speed. At first I thought it was just the air conditioner trying to keep up with the 95 degree weather, but that didn't make sense.
Took it in to the Chevy dealer today (I didn't know about using OnStar). They checked it out, and said that the engine intake temperature sensor was failing, so the car thought the ambient air was very hot and was trying to keep it cool by running the fan full speed.
They didn't have a new sensor in stock, so I'll have to live with a noisy Volt until it comes in. At least I can continue to drive it.
BTW, the service adviser told me they have a Volt loaner they reserve for Volt customers. That's pretty cool!
Edit: Just plugged my CodeKey in to the port, and saw that the error is P0111, "Intake Air Temperature".