My check engine light is on

Why my Check Engine light comes on?

The engine computer or ECM

All modern vehicles have a computer or the ECM (Electronic Control Module) that controls the operation of the vehicle powertrain (the engine and transmission). The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With constantly growing demands for better fuel economy and new strictest emission regulations it's not very easy to achieve. The engine parameters need to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, gasoline quality, ambient air temperature, road conditions, etc. That's why today's cars have much more electronics than in early days - there is a large number of various sensors and other electronic devices that help the vehicle computer or ECM to precisely control the engine and transmission operation and monitor emissions.
The vehicle computer system has self-testing capability. When the computer senses that there is a problem with some of the components it stores the correspondent trouble code(s) in its memory and lights up the "Check Engine" or "Service Engine Soon" light to tell you that there is a problem and your car needs to be looked at. To properly diagnose what is wrong, you need to take your car to a mechanic or a dealer. The technician at the dealership or a garage will then hook up the scanner to the car computer and retrieve the stored trouble code(s). Then he (she) will look it up in the service manual provided by a car manufacturer. The service manual contains the list of possible codes (about few hundreds) and describes what each code means and what needs to be tested. The code itself doesn't tell exactly what component is defective - it only indicates where to look, what engine parameter is out of normal range. The technician will have to perform further testing to pinpoint a defective part.
Here is how this works