The check engine light is part of your vehicle's onboard diagnostics (OBD) system. Computers have control and monitor vehicle performance, regulating such variables as engine speed (RPM), fuel mixture, and ignition timing, and may even tell the automatic transmission when to shift.

When the electronic-control system finds a problem it cannot adjust, the computer turns on a yellow warning indicator and stores a trouble code in it's memory. These diagnostic trouble codes (DTC) help identify the probable source of the problem, such as a malfunctioning sensor or a misfiring engine and can be read with a scan tool or diagnostic computer. Vehicle manufacturers originally used the OBD system to help technicians pinpoint and troubleshoot malfunctions. Exactly what the OBD system looks for depends on the make, model and year of the vehicle. 

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