Engine light and gas tank cap?

I have a 1997 grandam 6 cylinder 3100 engine.225000klm
Three days ago the check engine light came on and stayed
on. My mechanic said first check to see if the gas cap is loose. If it is, this can cause your check engine light to come on. It seemed a little loose and so I tighted
it.I then took it in to be looked and they reset
the computer and the check engine light went out
and stayed out all the next day, but on the second day it came back on and stayed on again.
The car is running fine except that the a/c stopped
working last week.
Does any of the above make sense to anyone?
Will disconnecting and reconnecting the battery
allow me to reset the check engine light to go off?
The mechanic did not tell me any code numbers
that may have shown up when they checked it
on the computer. They simply reset the code after
I told them that the gas tank cap seemed loose.
Could anyone please tell me what other things
might cause the check engine light to come on?
Thank You for any help