When a car has a fault it will usually show a warning light on the dashboard inside the car. If one of these lights is showing it is normally very easy to find the problem and fix it. However because of modern car technology, each new car has an onboard computer, which runs a number of different things from air-conditioning to satellite navigation.
With new cars it is sometimes a lot harder to find problems, so this is when an engine diagnostics computer and software are plugged in to a connection port found in the driver or passenger foot well. When the computer is plugged in, the software will run tests to find these hidden problems within the onboard computer and the cars engine.
Diagnostics with a computer on modern cars is highly effective as the computer can store previously hidden problems on a database so it is easily identifiable if another car of the same model has the same problem. However when finding a new problem it does take a while longer at is has to first rule out all other remembered problems that other cars have had.