The check engine light is turned on directly by the car's computer, which most
manufacturers call the PCM. This stands for power train control module. You may
find other names for this device, but basically it is considered the car's main
computer. The PCM monitors the many different automotive engine sensors. It
divides the sensors into multiple channels and monitors each channel and
compares the range of data to the specifications that are burned in memory. So
the check engine light on cars is turned on when the computer notices a
parameter out of range.
When the check engine light is turned on the
computer is notifying the driver that one of the channels that it is monitoring
is out of the specific range. When this light comes on the car may have set a
single code or multiple codes. An example of why the check engine light comes on
would be an oxygen sensor code. The range of operation that the computer is
looking for is between 50 and 800 mV. In this specific example, the computer is
also monitoring the changes in the voltage. If the oxygen sensor returns data
that is out of this range, the check engine light is turned on to notify the
driver of a problem.
Does this mean that replacing the oxygen sensor
would turn off the check engine light? The answer is in most cases, replacing
the sensor will not turn off the light. The reason that the check engine light
is on is because the sensor is out of range. This may be because the sensor is
faulty, but more than likely, the sensor is operating correctly and is out of
range due to an air fuel mixture problem.
This is where diagnosing the
check engine light properly will save you money and time in replacing
unnecessary parts. Using an automotive scan tool, you can view the data stream
for the particular sensors related to air fuel mixture. Another good rule of
thumb when diagnosing an oxygen sensor code is to diagnose the other codes that
are set, prior to diagnosing the oxygen sensor code. This is because the oxygen
sensor code may have been set, due to another system malfunction that affects
the data that the oxygen sensor and PCM monitor.
The above example of an
oxygen sensor code applies to most other individual sensor codes that could be
set in the car or trucks computer memory. This is why diagnosis is recommended
over just replacing sensors. Some automotive sensors are very expensive and all
are not returnable after installation. Also most automotive sensor can be tested
for proper operation before replacement is indicated. The auto scan tool is the
easiest way to properly diagnose and extinguish the check engine light. These
tools can be obtained for as little as fifty dollars and can pay for themselves
the very first time the light comes on.
Mark Gittelman is an ASE
Certified Master Technician With more than 24 years experience in the car and
truck repair business. More free information about
automotive scan tools is available at his auto repair and car questions web site
This author of this Article has choosen to make this article available with free reprint rights. Click here to copy this article.
Disclaimer: All information on this site is provided for informational purposes only! By no means is any
information presented herein intended to substitute for the advice provided to you by any health care or other professional
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