An engine diagnostic is a test which is performed to learn more about why an engine is not functioning properly. Engine diagnostics can be performed by skilled home mechanics with the proper equipment, or a car can be taken into a mechanic or dealer for diagnosis. The enginediagnostic is used to gather data which can be utilized in the repair of the vehicle, and it can also be used during things like emissions testing, in which a car is checked to confirm that it adheres to emissions standards for vehicles in its class.
Performing an engine diagnostic starts with plugging a device into the car's onboard computer. There is usually a space to do this just under the dashboard on the driver's side of the car. The device interfaces with the computer, returning a diagnostic code which can be referenced against a list of codes for vehicles from that manufacturer. If the engine is in good working condition, the device will inform the user. If there is a problem, one or several codes may be displayed.
Codes can mean a wide variety of things. During an engine diagnostic, the mechanic determines what is causing each code to display, for the purpose of developing an estimate to let a driver know how much it will cost to address a problem. Sometimes, the issue is simple, while in other cases, it can be very complex.