The check engine light flashes yellow when the electronic-control system of an automobile finds a problem it cannot adjust. The computer stores a trouble code in its memory to help identify the probable source of the problem. These trouble codes can be read by a scan tool or diagnostic computer.
The original system was not versatile as modern systems; it did little more than check whether the different electronic sensors were hooked up and working. Modern OBD II systems act like a built-in emissions monitoring station. Automakers are required to install this system in all modern vehicles as an article of federal law.