Last Sunday, while traveling about 65 MPH along I-88 in Illinois, the "Check Engine" light came on in my wife's 2010 Prius. She has about 6,000 miles on it. At first, I thought perhaps I hadn't turned the gas cap securely enough so when we got home, I filled the tank and closed it securely; sure enough, the light went out the next time I started the car. We were both happy that this appeared to be the cause but the next day, the light came on again and has not gone out. I have an appointment with Toyota Friday to run a scan to find out what error code is showing up. Anybody have any ideas about the most common causes, especially on such a new car, might be? My first guess would be some sort of emission control failure on one of the components but what other common reasons are there?