Engine Indicator Light

My mom owns a 2002 Chrysler Sebring and the EIL is on steady. She took it to Dobbs and they ran a diagnostic series on it and told her that her gas cap was not on tight enough. The tightened it and the light went out. Two days later, the light came back on and she took it back and they told her that the diagnostic test was ran again and there was nothing wrong with her car. She went and got a new gas cap, had it put on and the light went out. One week later the light came back on and is steady burning. The problem is that she will have to have the emission control testing done and has been told that the car will not pass because of the light. What can we do to get the light to go off and stay off?