All modern vehicles have a computer or the ECM (E
odule) 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.
computer or ECM
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.