Ok, so I realize that alot of us, particularly individuals new to working on their cars, often have difficulty understanding the complex role the ECU, its sensors, and various systems play to keep things running smoothly.

To that end, I was reading my Helms tonight, as I often do, I stumbled across this simple, but clarifying graphic showing the relationship between these systems.



Beginning in the far left box we have a list of various sensors attached to the engine and drive-train which communicate to the ECU through voltage the current status of the particular system they monitor.

In the middle box, primary ECU responsibilities, or vehicle operations that it is responsible for managing efficiently.
The criteria for how these systems are managed are quite detailed and far beyond the scope of this post.
If you want to know more about fuel and ignition maps I suggest heading over to pgmfi.org and reading awhile.

On the right are the "tools" or specific components on the car the ECU can manipulate to keep the vehicle running properly.

For example, take the oxygen sensor, abbreviated HO2S.
It's function is to report back to the ECU the amount of oxygen content in the vehicles exhaust stream.
This information is sent to the ECU as a measure of voltage and is used to calculate appropriate duty cycles for the injectors to dispense fuel.

This example is just one sensor and a single responsibility the ECU has to keep tabs on at any given time.
The entire time your vehicle is running the ECU is constantly gathering information from all of your vehicles sensors and when necessary, making adjustments to remain efficient as possible.

When you see that annoying CEL or check engine light turn on on your dash it is because one of the sensors in the left hand box has either sent a signal outside of the acceptable input range, or has stopped reporting altogether.

I hope this helps the new car guy get a better hold on how these systems operate.