check engine light comes on but doesn't stay on; can't get code

Hello, when the check engine light comes on, a diagnostic trouble code will be stored in memory of the engine's processor until one of three things happen -- either the code is erased by a diagnostic tool, a code clears after a certain amount of drive cycles -- not usually less than 3 or 4, or the battery loses power. Is the check engine light coming on while driving? What is being done to retrieve codes?