Vehicles manufactured since 1974 have a variety of emission control systems. As the engine control and monitoring systems became more complex, on-board diagnostic systems were developed to record fault codes and assist in engine problem diagnosis. Beginning with the cap on the gas tank through the engine and out the muffler, something out of tolerance will generate a fault code that is recorded.
In general, all vehicles equipped with on-board diagnostic systems (OBD-II is the current standard) will store error codes that can be read using a portable code reader. A "Check Engine" light comes on to indicate that the engine management computer has detected a problem related to emissions or engine operation. It is your notice that something needs to be fixed in order to comply with federal emissions requirements or to keep your engine running.
Many of the trouble codes relate to engine or emission sensor inputs that have failed or are out of tolerance. The sensors control the "engine management computer" and keep the engine running at optimum efficiency. Failed or out of tolerance sensors mean your engine is not operating at optimum efficiency.
While it is easier to read the fault codes with a code reader/scanner, most vehicle provide an option to display the codes.
To view the codes, start with the engine turned off. Turn the ignition key to the "ON" (not "IGNITION/START") position, then turn it off for about a second. Turn it on for a second, then turn it off for a second. The third time you turn it on, you should see a set of trouble codes flashing on the odometer display. (NOTE: this process works with most newer Jeep vehicles. It may or may not work with other models.)
Once the error codes are recorded, interpretation will indicate the likely source of the fault. Fixing the problem can be simple do-it-yourself (replace the gas cap) or more complex requiring the services of an auto repair shop.
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For a comprehensive listing of ODB-II error codes for all manufactures vehicles, check these websites: