Modern cars are complicated machines and like most modern machines are computer controlled. Within a vehicle there will be at least one computer responsible for engine management, but often there will be multiple computers designated to control different parts of the vehicle such as climate control, security etc. All of these computers have a common communication line and send information around a network thousands of times each second. This is why modern cars are so much more efficient than older cars, because things can be precisely controlled. However like all things electronic sometimes a failure will occur in either the computer or the sensors it used to monitor performance. In this case a device known as a scan tool is used to communicate with the vehicle to extract information. This process is what is often referred to as scan tool diagnostics.