All modern vehicles have a computer or ECU (Electronic Control Unit) that controls engine operation. The main purpose of this is to keep the engine running at top efficiency with the lowest possible emissions. With today's strict emission regulations it's not very easy to achieve. The engine needs to be constantly and precisely adjusted according to various conditions such as speed, load, engine temperature, fuel quality, ambient air temperature, road conditions, etc. A number of sensors provide the ECU with all of the necessary information to run the engine at best performance under any situation. According to these inputs, the ECU makes adjustments like adding or subtracting fuel, advancing or retarding the ignition timing, increasing or decreasing idle speed, etc.

There is also an oxygen sensor installed in the exhaust before the catalytic converter that monitors the quality of combustion in the cylinders. Based on the feedback from this oxygen sensor the ECU makes fine adjustment to the air-fuel mixture to further reduce emissions.

The ECU has a self-diagnostic capability and constantly tests the operation of sensors and other components. When any of the sensor signals is missing or out of normal range, the ECU sets a fault and illuminates the "Check Engine" or "Service Engine Soon" light also called MIL (Malfunction Indication Light) storing the corresponding Diagnostic Trouble Code (DTC) in the ECU memory

The same will happen if a mechanical component of the controlled system fails. For example, if the EGR valve fails, this will also cause the "check engine" light to come on.

The stored trouble code can be retrieved with a special diagnostic tool (Scan tool). The code itself does not tell exactly which part to replace, it only gives a direction where to look for. It is then up to the Technician to use his knowledge and experience to correctly identify the faulty