Car diagnostic codes identify problems that onboard systems report during the operation of the car, according to Total Car Diagnostics. Sensors in the car note and record readings that vary from the accepted range, such as a fuel mixture being too rich.
The diagnostic system within the onboard computer stores codes every time the sensors identify a potential problem. The car's owner's manual indicates the significance of each of the codes. Repair professionals and do-it-yourself car repair experts use the codes to determine the proper circuits, system or components for further testing. Codes result from a problem somewhere in the system that a particular sensor monitors and are not necessarily due to a problem with the sensor itself, notes Total Car Diagnostics.