Read a vehicle's on board diagnostic code, or ODC, by attaching a code reader to the port under the steering wheel and starting the car. Compare the reading to a code guide available in a repair manual or online. Some scanners provide potential problems in the readout along with the code.
The ODC is an alphanumeric code that helps to narrow down the possible causes of problems with a vehicle. In most cases, the on board computer causes a check engine light to illuminate, warning the driver that something is wrong with his car. While each code indicates a range of different problems, it is a starting point in diagnostics.
With scanners that provide only the code, the mechanic must complete further tests to determine the actual problem that is causing the check engine light to illuminate. Mechanics sometimes use scanners that provide the code along with real-time information that are identical to the ones that the sensors send to the on board computer, helping to eliminate the need for further testing.
Many automotive parts stores have code readers and offer a free service to provide vehicle owners with the code reading. However, car owners who perform much of the work on their vehicles sometimes benefit from purchasing their own scanners.