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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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