Drew writes “If your vehicle was manufactured after 1996, it contains an On Board Diagnostic (OBD) computer that captures information about how it’s running. I’ve put together a basic HOWTO Guide for OBD-II so that you can get started playing with it. I use ScanTool‘s great hardware and GPL’d software for my setup. You can use it to save money by reading error codes without a trip to the mechanic when your “check engine” light is on, capture performance data about your car’s drivetrain in real-time, or log data as if it were an airplane’s “black box”.” Link.