They sell gadgets that you can plug into your car that allow you to read the code from the car's computer that caused the check engine light to come on and clear the code (which turns off the check engine light). I've seen them for as low as about $60. Some of the fancier gadgets that can do the same thing will give you a lot of interesting information from the onboard computer, like instantaneous MPG, engine RPM, coolant temperature, engine loading, horsepower, along with having useful trip computer information. The one I have is a ScanGauge 2, which runs about $150 or so, and has all the fun trip-computery features.