Vehicles manufactured since 1974 began sporting a variety of engine and 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 problem diagnosis.
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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 vehicles provide an option to display the codes without a scanner.
To view the error codes, start with the engine turned off (Ignition Key in the OFF position). 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, trouble codes should begin flashing on the odometer display. (NOTE: this process works with newer Jeep vehicles. It may or may not work with out models.)
Once the error codes are recorded, interpretation will indicate the likely source of the fault.
For a comprehensive listing of ODB-II error codes for all manufactures vehicles, check these websites: