Car code readers can only be used on OBD II (on-board diagnostic two) systems integrated in all vehicles since 1996. Some GM models began OBD II in 1995. OBD II standardised the location and plug application of the vehicle's computerised system, which simplified the equipment needed and the diagnosis time. The first OBD II scanners were expensive and more complicated to use because they required different data chips for various car models. The pocket code readers today require no data chips and are much more affordable to the home mechanic.
Open the driver's-side door and look under the dash for the trapezoidal diagnostic link connector (DLC). Since the implementation of OBD II, it is mandated that all DLCs be located within two feet of the steering wheel. Most are located under the driver's-side dash on either side of the steering wheel column. However, this is not the case for some models. Refer to the owner's manual of the vehicle if necessary to locate the DLC.
Remove the DLC cover if there is one. Some vehicles don't employ DLC covers. The covers simply pull off.
Insert the code reader's plug into the DLC outlet. This outlet is easily identified, since it is similar in shape to the code reader's plug. The vehicle's battery will provide power to the code reader.
Turn the ignition key to the "on" position without starting the engine. This will provide power to all electrical components and illuminate the instrument panel lights.
Read the menu of the code reader and use the key or arrow to move to the "read codes" or "DTCs" (diagnostic trouble codes) option. Since code readers are made by many manufacturers and feature slightly different options, you will need to read the manual of the code reader to fully understand all of its functions.
Press the "enter" key, if applicable, once the "read codes" or "DTCs" option has been selected. Refer to the code reader manual to determine which DTC triggered the malfunction indicator lamp (MI, also known as the check engine light or service engine light).
Clear the code, if applicable and desired (some inexpensive models simply read codes), by scrolling to the "erase code" selection and pressing enter. Some code readers may feature an "erase" button which simply needs to be pressed.
Wait until the "command set" message appears on the onscreen menu of the code reader.
Many newer versions of code readers are multifunctional and will walk you through the computer's self-diagnostic test (also known as IM monitors reset). This procedure requires driving the vehicle for several miles or several driving cycles (where the vehicle runs for a certain amount of mileage and experiences operating temperature and cool-downs) while able to monitor the computer's self-diagnosing progress. Once the monitors are reset, the MIL will either remain out because the DTC was corrected by the proper repair, or it will illuminate because it has again detected the problem.
Code readers should be used to diagnose the problem that triggered the MIL light. The problem should then be fixed. Once the problem is corrected, you can wait until the computer re-diagnoses itself and if the problem is indeed fixed, it will reset itself.