The CEL will not go out as soon as you fix what is indicated as being bad. The computer has to go through some cycles to convince itself that the error no longer exists. If you have access to an OBDII scanner that can reset a CEL then you would be able to turn it off immediately.
From my '99 Owner's Manual:
What you should do if the check engine light illuminates
Light turns on solid:
This means that the OBD II system has detected a malfunction. Temporary malfunctions may cause your Check Engine light to illuminate. Examples are:
1. The vehicle has run out of fuel. (The engine may misfire or run poorly.)
2. Poor fuel quality or water in the fuel.
3. The fuel cap may not have been properly installed and securely tightened.
These temporary malfunctions can be corrected by filling the fuel tank with good quality fuel and/or properly installing and securely tightening the gas cap. After three driving cycles without these or any other temporary malfunctions present, the Check Engine light should turn off. (A driving cycle consists of a cold engine startup followed by mixed city/highway driving.) No additional vehicle service is required.
My '99 Mounty used to throw a CEL for loose gas cap every other tank of gas. (Here in Oregon the drivers aren't smart enough to fill our own gas tanks, unfortunately the trained experts who pump our gas for us don't know how to put gas caps back on correctly.) About 160 miles into a tank of gas the CEL would come on. I would remove and reseat the gas cap and it took about 160 miles for the CEL to reset itself.
One time as I was reseating the gas cap I noticed a fine crack in the filler tube. I put some superglue into the crack, clamped it and let it cure overnight. No CEL in about 10 fill ups.
