The check engine light on cars is a very sensitive warning device.

The warning 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 senors marking high and low values and changes in the data stream.

It then divides the sensors into multiple channels and monitors each channel and compares the range of data to the specifications that are burned in memory.

When the service engine soon 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 light? The answer is in most cases, replacing the sensor will not turn the light off.

The reason that the service 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 malfunction properly will save you money and time in replacing unnecessary parts.