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.
Why the check engine light comes on.
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 R Gittelman has more than 23 years experience in the auto repair field. He holds 16 ASE certifications and is a Master Technician in cars and heavy-duty trucks. Visit his car questions website for more valuable do it yourself auto repair help. Or learn more about how to reset the check engine light in any car or truck.
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