Service Engine Soon
Using your car's on board diagnostic system


A powerful computer controls a broad range of functions throughout your car's engine and power train. This on-board computer uses an extensive set of sensors, located throughout the automobile, to closely monitor systems operation many times each second. This computer engine control system was originally designed to enable automobile engines to meet air pollution standards established by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) while increasing automobile fuel efficiency.

To affect engine operation, the computer controls devices, called actuators, such as fuel injectors, idle air control, exhaust gas recirculation (EGR) valves, and the ignition module. To help reduce air pollution, it also monitors and records information related to engine emissions. When certain problems are detected, the system lights the Malfunction Indicator Lamp (MIL) and records Diagnostic Trouble Codes (DTC's).

All cars sold in the United States since 1996 are required by law to include a computer control system meeting requirements known as On-Board Diagnostics, Generation Two, or OBD II for short. These cars have a standard electronic cable connector, often located under the dash near the steering column. Various devices, called code readers or scanners, can be plugged into this connector to read the diagnostic scan codes, and also turn off the MIL.

The “Malfunction Indicator Lamp” (MIL) is the accepted term used to describe the lamp on the dashboard that lights to warn the driver that an emissions-related fault has been found. Some manufacturers may still call this lamp a “Check Engine” or “Service Engine Soon” light.

After the MIL has been turned off, DTCs, Freeze Frame data, and manufacturer-specific enhanced data stay in the computer’s memory. Most of the enhanced data can only be retrieved with special equipment such as a Scan Tool.

The diagnostic trouble codes give important clues about the particular failure that caused the MIL to light. Often these trouble codes can lead a skilled person to a successful repair of the problem.

Fix any known mechanical problems before performing any test. See your vehicle’s service
manual or a mechanic for more information.

service engine soon, mic light, auto repair, obd II, on board diagnostics,code reader © 2005 Leland R. Beaumont