How to Use Car Scan Tools
Popular Mechanics has a good item on "How to Use an Auto Scan Tool" (Mike Allen, July 2008). This describes how to use low cost On-Board Diagnostics (OBD-II) devices plugged into the car to report diagnostic trouble codes (DTCs):
To get started, plug the scan tool into the OBD II connector under the dash. Turn the key on, but don't start the engine. The tool will ask for a number of things such as the VIN, the make and model of the vehicle and the engine type. Follow the onscreen instructions.
You'll get an option to check for trouble codes, as well as a couple of other menu choices. Some higher-end scan tools give you a text explanation of the code onscreen. ...
From: How to Use an Auto Scan Tool", Mike Allen, Popular Mechanics, July 2008