1997 Other Kia Models Check engine light

My KIA Sephia's check engine light came on a couple of years ago. I asked a mechanic about it and he said to take off the gas cap and re-tighten it. This worked at the time. We had the car parked for a couple of years and started running it again. The light is back on and the gas cap trick doesn't work. The mechanic said that KIA's are very sensitive and not to worry uless the light was flashing. The car seems to run fine and gets great mileage still. So how do I turn off the light?