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Fixing Your "Check Engine" Light
By Pat Goss
Printable version
Degree of DifficultyModerate
Moderate
Estimated Time60 minutes
60 minutes
Getting an accurate yellow “Check Engine” or “Service Engine Soon” light diagnosis is often frustrating and expensive. Fortunately, if you happen to be technically inclined and have the basic tools, it is likely that you can do the job yourself and avoid that expensive repair shop visit.
 
Designed to alert drivers to computer-monitored emissions problems, the “Check Engine” light is actually part of the vehicle’s emissions system. “Check Engine” lights became a standard equipment feature when automotive on-board computers proliferated in 1981. Federal law says that every new car sold in the United States must have a “Check Engine” light. But after more than two decades, “Check Engine” lights remain a mystery to many technicians and automotive do-it-yourselfers.
 
Here’s an ultra-simplified version of how the “Check Engine” light works. Vehicle computers use input signals from sensors to generate control signals for fuel, spark delivery, transmission shifting, and other functions. The car’s computer continuously monitors all input signals that could effect emissions. If any of the monitored signals move outside government-mandated limits, the computer turns on the “Check Engine” light.
 
The computer also determines if the problem meets the criteria for setting a code. However, rather than identifying a part or system that has failed, these codes refer to the part or system that is being affected by what has failed, making them more confusing than helpful.
 
Because this code system can confound even experienced technicians, it often results in unnecessary repairs. Oxygen sensors, for example, are extremely reliable, yet millions are needlessly replaced every year, largely because of the wide array of problems that cause the computer to set an oxygen sensor code.
 
Oxygen sensors examine the amount of oxygen in the exhaust gas leaving the engine. The sensor compares the oxygen inside the exhaust system to oxygen in the air outside the sensor. Rich exhaust has less oxygen; lean has more. The amount of oxygen in the exhaust is directly related to the fuel/air mixture entering the engine. A rich incoming fuel/air mix produces exhaust with less oxygen, while lean produces exhaust with more oxygen.
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