To read engine diagnosis codes, plug the code reader into the 16-pin OBD II diagnostic connector (typically found under the dash by the steering column), then turn the ignition on, but do not start the engine, according to Popular Mechanics. Press the Read or Start button to display the codes.
At this point, the code reader should display any current diagnostic fault codes in numerical order. Some high-end scan tools also show an explanation of the code; if the reader in question doesn't, the codes can be searched at sites such as Actron.com.
Most codes only indicate a particular sensor circuit or system has a problem; it will not tell the reader what part to replace. Reader owners need to diagnose the faulty part that needs to be replaced to permanently fix the problem. Write down the codes for future reference, then reset the malfunction indicator lamp (MIL).
The diagnostic codes on all OBD II systems are generic, which means all vehicle manufacturers use the same common code list and 16-pin diagnostic connector. However, each manufacturer also has the freedom to include their own "enhanced" codes to include more detailed information about different fault codes. These enhanced codes also cover failures not related to emissions that occur beyond the engine control system, including ABS, HVAC and airbag codes.