How does OBD-II inform drivers of problems?
When the OBD-II system determines that a problem exists, a corresponding “Diagnostic Trouble Code” (DTC) is stored in the computer’s memory and a special lamp on the dashboard called a Malfunction Indicator Lamp (MIL) is illuminated. This lamp is reserved for emissions problems only and cannot be used for other failures. Automobiles use a variety of warning lamps to notify drivers of different conditions. In the case of an emissions MIL, either the phrase “Service Engine Soon” or “Check Engine” is used or an engine symbol is displayed. This light, usually yellow in color, serves to inform the driver that a problem has been detected, and vehicle service is needed. When the car is delivered to the repair shop, a service technician can quickly retrieve the stored diagnostic trouble codes from the computer memory of the vehicle. The stored diagnostic trouble code(s) will help the service technician diagnose and repair the problem. It is important to note that an illuminated MIL is intended
