An illuminated check engine light on a vehicle's instrument panel indicates a potential malfunction within the vehicle's systems. The light is triggered by an out-of-range value on one or more sensors read by an on-board diagnostic (OBD) computer.
As modern vehicles became increasingly complex in the 1980s, many manufacturers added a centralized, diagnostic microprocessor. Early versions of this computer, called OBD-I, were not standardized. The Clean Air Act of 1990 mandates that vehicles sold in the U.S. after 1995 include an on-board diagnostic computer with standardized codes. This second generation is termed OBD-II.
All vehicles with a second-generation, diagnostic capability continually monitor the status of the power train, particularly the fuel consumption emission controls, as mandated by the Clean Air Act. Different manufacturers also include real-time monitoring of other systems such as body and chassis integrity.
When any sensor reading is outside its normal range, the OBD computer illuminates the check engine light. The processor also captures a five-character code that specifies the condition. There are several hundred codes for a vehicle's power train plus manufacturer- and model-specific codes for other systems. Mechanics use a device called a code scanner to determine the problem by reading the code. Most scanners also reset the check engine light.
It is not possible to determine what problem has triggered a check engine light without querying the OBD computer with a scanner or other diagnostic tool. The normal meaning of the check engine light is, "Take your vehicle to a mechanic as soon as possible." Consult your vehicle's owner's manual for specific instructions.
