Scan Tools
Automotive scan tools are computerized devices that plug into the computers commonly found in cars these days. Also called diagnostic scan tools, these devices read information stored in your car's computer to determine if the car is malfunctioning and in what manner. The auto scan tool then prints out, or displays on a screen, a report of its findings and recommendations for adjustments or repairs.
The need for scan tools arose from concern for the environment. In the early 1970s, the U. S. Environmental Protection Agency (EPA) mandated the installation of On Board Diagnostic (OBD) computers in all cars driven on U.S. roads. The idea was to enable collection of data about an engine's performance and air pollutant emissions, so that cars could be kept running cleanly. The formal term for scan tools is "OBD scan tools." They literally scan the OBD computer of your car. (Note that's OBD, not ODB!)
Most cars now have a "MIL" light on the dashboard. "MIL" stands for "Malfunction Indicator Lamp" and is used by the OBD computer to tell you that you need to get the OBD scanned. The MIL light is commonly labeled and known as the "Check Engine" light. When you see that message, there's something wrong with your car that you should not ignore for very long.
But in other cases, the "Check Engine" light may indicate a very minor condition that doesn't need a professional mechanic to fix. How can you tell the difference? By reading the code yourself with an OBD scan tool.
Even a cursory diagnostic inspection by a professional car mechanic can cost nearly $100. Fortunately, there are scan tools you can buy for under $60 that will tell you what's wrong with your car. In some cases, you can repair the problem simply by resetting the car's computer to its factory-default settings. Some scan tools enable you to do this, so you can save money and down-time.
The bits of data that a scan tool reads are called Diagnostic Trouble Codes (DTC). Each code is a string of bits that stands for a specific condition, e.g., "cylinder firing mistiming." The scan tool reads all the DTCs in your car's computer and reports them to you. Cheaper scan tools simply report the raw codes, and you have to refer to a book to figure out what they mean. Better scan tools store the meanings of DTCs in non-volatile memory and tell you, in more or less plain English, what the trouble is.
OBD scan tools won't make you a qualified car mechanic, but they can give you an idea of what the trouble is before you go to one.