Engine Light Warning

The check engine light can be very sensitive warning device. In this quick article I will discuss why the check engine light comes on. I'll also discuss a few ways to diagnose codes and in turn extinguish the check engine light.
The check engine light is turned on directly by the vehicle computer. Most manufacturers call this the ECM. This stands for Electronic control module. You may find other names for this thing but basically it is considered the vehicles primary computer.
The ECM watches many different automotive engine sensors. It divides these sensors into multiple channels and monitors each channel and compares the range of data to the specifications that are burned in memory. The computer is monitoring both inputs like coolant temperature and oxygen sensors but also out put controls like idle air control motor steps and egr pulse width.

So the check engine light is turned on when the computer notices a parameter out of range. Keep in mind that output control parameters like the EGR and IAC mentioned above are usually measured with a resistance value or reference voltage to assure proper component operation.
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 many codes.
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 that set the code.
Another good rule of thumb when diagnosing sensor codes is to diagnose the failures that are set with lower numbers, prior to diagnosing the other sensor codes. This is because the higher number codes could be set by the lower number failures. You can also apply some common sense to this as in a misfire code should be addressed before repairing a rich exhaust code. Since the UN burnt fuel from a cylinder is detected as a rich condition.
To review your computer is watching multiple channels and multiple sensors and simply comparing the data with the parameters that have been programmed into the unit. Above or below these data settings commands the ECM to notify the driver they are polluting the atmosphere by turning on the check engine light.
In some cases these out of range readings can be either temporary or intermittent. This is again where the scan tool comes in handy for figuring out why the check engine lamp is illuminated. You can clear the codes and run the vehicle and see if the check engine light comes back on. If it does, then you have a path of diagnosis related to the diagnostic codes that have returned.