Doug DeMuro of Jalopnik explains the problem with the check engine light. (Experienced drivers solve it with a strip of electrical tape.)
So what happens, when you have a check engine light, is as follows. The first time you see it, you freak out. "OH MY GOD!" you say. "A CHECK ENGINE LIGHT! MY CAR IS ABOUT TO BLOW UP!!" So you cancel all your appointments, and you tell your clients to find their own rides to the airport, and you tell your husband that he'll have to pick up the kids today, and you rush to the dealer, convinced your engine is moments away from a Hollywood-style explosion that will make the evening news. And you get there, and the mechanic plugs in his tool, and he charges you eighty bucks just so he can tell you that this scary, ominous warning light came on because you stopped turning your gas cap three millimeters too early.