The idle air control valve. I have a little over 121,000 miles on the car. Can't remember when I had the fuel filter replaced, but the plugs, cap and rotor all have been replaced recently.

The reason why I think there is a "switch" is because a friend of mine that helps me out with my car fiddles with the IACV (it causes the CEL to come on) to get my car started due to reason 1. I let the car idle for awhile just to get the motor running for a while. But the next time that I turn on the car the CEL is not on anymore. He suggested that I might have a bad IACV and that it might have to be replaced.

Looking things up, I see a lot of suggestions that the IACV can just be cleaned rather than be replaced, but I wanted to know a little more about it (about how the IACV goes bad).