It simply means that something is wrong with a computer in your car. It could be anything from a loose gas cap to a rodent-chewed wire. There are quite a few sensors in your car that can turn the light on (cam shaft position sensor, manifold absolute pressure sensor, throttle position sensor, mass airflow sensor, oxygen sensor...). The severity of the problem varies, but should be looked at as soon as possible. A flashing check engine light indicates a misfire.
When the OBD II system detects a problem, a "Diagnostic Trouble Code" (DTC) is stored in the computer's memory. The computer also illuminates a dashboard light indicating "Check Engine" or "Service Engine Soon" or displays an engine symbol. This malfunction indicator light can only be used to indicate an actual problem detected, and not for simple reminders for maintenance.
An illuminated malfunction indicator light indicates that a problem has been detected and alerts the operator to have the vehicle serviced as soon as possible. Certain severe engine malfunctions may cause the malfunction indicator light to blink or flash, and this indicates a need to reduce speed and to seek immediate service.
When the vehicle is presented for repair, the technician will retrieve the stored diagnostic trouble codes from the vehicle's OBD II computer using a "scan tool." The information obtained will assist the technician in isolating the particular systems having the problem. It is important to note that, while the OBD II system may identify specific systems and certain operating condition information, these are often only part of the job. It may still take diagnostic skill, knowledge and additional research to determine the actual cause of the problem. For example, the OBD II system may identify a problem with a sensor circuit, but the technician must still perform specific diagnostic steps on that system to determine whether the sensor itself is at fault, or a loose connection or even the vehicle's computer is causing the problem or whether the component that the sensor monitors is defective. The only way to do that is to have the proper equipment, reference material and training. Some facilities build their diagnostic time into the overall labor charge, and some charge diagnostic time separately. It is important to understand that diagnosing any emissions problem can take time, and charging for that time is appropriate.
