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Fixing Your "Check Engine" Light
By Pat Goss
Printable version
Degree of DifficultyModerate
Moderate
Estimated Time60 minutes
60 minutes
To understand how the oxygen sensor knows the difference between rich and lean, think of it like you would a battery. Oxygen sensors contain a crystal that is sensitive to oxygen. These crystals produce a small voltage when there is more oxygen on one side of the crystal than the other. When the mixture is lean, there is a small difference in oxygen content between the exhaust and the atmosphere, so the crystal produces a low voltage. The exhaust from a rich mixture has less oxygen than is in the air, so the crystal produces a higher voltage. Normal voltage ranges from one hundred millivolts (lean) to nine hundred millivolts (rich).
 
Voltage produced by the oxygen sensor goes directly to the vehicle’s computer. When the mixture is rich and the sensor voltage is high, the computer reacts by commanding a lean mixture. The lean mixture produces more oxygen in the exhaust and the voltage drops, which causes the computer to command a rich fuel mixture. As a result, the fuel/air mixture constantly fluctuates between rich and lean. The rich-lean cycle repeats many times per second, producing an average fuel mixture halfway between the two extremes.
 
What does this have to do with codes? The oxygen-rich excess air can’t burn completely and leaves the engine with the exhaust. Excess oxygen in the exhaust causes the oxygen sensor to always produce the same voltage and eventually quit switching from rich to lean. Without fluctuating voltage from the oxygen sensor, the fuel mixture locks up. The computer monitors how much and how often the voltage changes, and if it doesn’t meet specifications, the computer turns on the light and sets a code. Unfortunately, the computer can’t determine why the sensor isn’t switching from rich to lean, so it is limited to setting an oxygen sensor code.
 
This approach routinely creates expensive problems, because most people facing an oxygen sensor code install a new sensor, clear the code, and assume the car is fixed. But the “fix” is usually short-lived, because nothing has really been fixed. The underlying problem still exists.
 
Sooner or later, the code is activated again. Why? Clearing codes also erases the computer’s monitors. These sub-programs in the computer constantly look at individual circuits, and unless all monitors are “Set and Ready,” the “Check Engine” light cannot come on. The monitors will not reset until a specific series of driving conditions has been completed.
 
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