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An illuminated engine warning light means your vehicle has detected a potential emissions fault.The computer has logged one or more diagnostic trouble codes that correspond to the problem and turned on the warning lamp to alert you to the problem. There is NO way to determine the nature of the problem without connecting a diagnostics computer to the vehicle's diagnostic connector to read the fault code(s). Once this has been done, further diagnosis and testing may be required to isolate the fault so the correct parts can be replaced. Depending on the nature of the fault, your engine may not run as good as it normally does, or it may use more fuel than usual. But usually the problem requires attention as soon as it is possible. You can continue to drive your car until it can be diagnosed.
Common reasons for the CHECK ENGINE light to come on include the failure of an engine sensor such as the oxygen sensor, throttle position sensor or manifold absolute pressure sensor, or a problem in an emissions control system or such as the catalytic converter. Oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. They determine if the air fuel ratio exiting a gas-combustion engine is rich (with un-burnt fuel vapour) or lean (with excess oxygen). Un-burnt fuel is pollution in the form of air-borne hydrocarbons, while oxides of nitrogen (NOx gases) are a result of excess air in the fuel mixture and cause smog and acid rain. In practice the oxygen sensor continually signals the ECM to alter the fuelling by a small amount, to maintain the mixture strength as near as possible (+- 1%) to the theoretical ideal (stoichiometric) ratio of 14.7 parts air to 1 part fuel, which is the optimum for the catalytic converter to work efficiently.