2000 Hyundai Sonata Check Engine Light

I take good care of my Sonata, since I am a courier, and put 150,000 miles a day on it. Recently, the check engine light came on, I had the code read at AutoZone, got a "change your gas cap" kind of result, but I detect no difference in the engine's performance(unlike the last time, when I replaced the fuel/air sensor-right,down, middle side of the engine block). A friend of mine, who is a backyard mechanic, says that Hyundai has programmed their automotive computers to turn on the check engine light at certain mileage intervals, and it won't go off, or, the computer will start making the engine run stupid, until certain preventative maintenance things are done(tune-up, cooling system flush, sensors replaced,etc.). He claims the computer won't reset unless all needed procedures are done, and they have to be done all at once, at the same time.
This sounds like hogwash to me.
What do you know about this?