On all modern vehicles there is a computer that controls how the machine is operating, the computer is called the electronic control module, or ECM. The goal of ECM is to keep the engine running within the limits emissions and at peak efficiency. With very strict emission regulations at this time, this is not an easy task to achieve. Precise and constant adjustment to suit the various conditions of the engine must be performed by ECM in terms of speed, load, engine temperature and others.
How ECM works:
A number of sensors available that provides ECM with the information needed, such as the engine and temperature inputs, vehicle speed and load. ECM to make adjustments advancing or inhibiting the ignition timing, fuel increase or decrease or increase and decrease the idle speed.
In the exhaust, before the catalytic converter, oxygen sensor monitors the combustion quality is produced in the cylinder. Sensors generate feedback is then used by the ECM in order to smooth adjustment to the fuel-air mixture in order to reduce emissions. Once the catalytic converter, the other sensors that monitor exhaust efficiency
There are other additional emission control systems, such as evaporative system, or EVAP. EVAP goal is to prevent gasoline vapors from the gas tank, from being released into the atmosphere. There are also a number of sensors and actuators are controlled by the ECM.
The operation of sensors and other engine components are continuously tested by self-diagnostic capabilities of ECM. If one of the signals is lost, or if it is not within the normal range then check engine light is on and the diagnostic trouble codes, DTC, is stored in the memory of the ECM. Check engine light will be illuminated as well if the mechanical components, such as EGR valve, failed.
With a special tool scans, the stored code can be accessed by a technician, who then would know just which way to look. The code does not tell the specific information about the failure, certain tests should be performed for each code, to find the exact cause of the problem.