The check engine light on the instrument panel of all cars sold in the United States since the mid-1980s is connected to a diagnostic computer that monitors the engine and other systems, especially emissions control. If one of the diagnostic sensor readings is outside a specified range, the computer turns on the check engine light. The computer also captures a code identifying the bad reading.
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Computer monitoring using on-board diagnostic (OBD) processors began in the 1980s. The Clean Air Act of 1990 mandates that all vehicles sold in the U.S. after 1995 be equipped with an OBD system using a standard set of codes. The system is called OBD-II (on-board diagnostic, generation 2) in the industry. The codes captured by an OBD-II computer can be read with a code scanner.
The diagnostic system, whether OBD-II compliant or using the older, non-standardised, OBD-I protocol, continually monitors an array of sensors. When a sensor reading is found to be "bad," the OBD computer illuminates the "Check Engine" light and stores a five-character code in memory. For a post-1995 Toyota Corolla, the OBD-II code can be read with a scanner. Pre-1996 Corollas require that a mechanic read a "blink code" to determine which sensor triggered the light.
A lit Check Engine light is a general warning that your Corolla should be serviced as soon as possible. Consult the owner's manual for specific instructions. Without a scanner or a blink code reading, it is impossible to determine the problem or its severity.
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