On-board computers have revolutionised automotive repair by monitoring various systems, mainly emissions-related, in the vehicle's drive train. As the computer gathers and stores data it receives from various sensors throughout the vehicle, it compares that data to presets programmed into the computer at the factory. If the data does not match, an error code is set and the check-engine light comes on. While there is more to the diagnosis process than simply retrieving the codes, knowing what the codes are is the first step in fixing the problem.
Look under the driver-side dashboard for the diagnostic trouble code (DTC) connection port. It is black, rectangular-shaped and roughly one inch wide by a 1/4-inch tall. It should have a small cap on it to prevent dirt from entering the connector. Quite often the cap will have the letters "DTC" embossed on it.
Remove the cover from the connection port and move the cover off to the side. It should be attached to the port by a thin strand of plastic to prevent the cap from being lost.
Plug in your code reader's data link cable to the connection port.
Turn on your code reader and refer to your specific reader's instruction manual for the procedure to pull diagnostic codes.
Write the codes down, once you have them, as you will need to refer to them during the troubleshooting process.
Contrary to what many might have you believe, the code itself does not tell you the problem your vehicle is experiencing. The codes correspond to troubleshooting pinpoint tests, which, when performed sequentially, will lead you to the cause of your problem. For example, if you get a P1408 code, which means "EGR out of self test range", that does not necessarily mean you have a faulty EGR valve. Any number of malfunctions can cause the EGR valve to behave improperly. Knowing the codes is only one step in the bigger picture of automotive repair.