When the Service Engine Soon light comes on, it's the on-board computers way of telling you it sees a problem in one of the monitored systems. If you are not experiencing drivability issues (surging, stalling, rough idle), then more than likely it is a problem in an emissions system. Unfortunately, there are litterally 100s of sensors monitored and to take a "guess" would just be inefficient and a waste of time and money. A good place to start is at your local parts store (ie. NAPA, AutoZone, Advanced). Most of these stores will read the "p-codes" from the on-board computer for Free in hopes of selling you the part/sensor you need to make the repair. Once you have this p-code, it will tell you which system is faulting, and usually which sensor/part is the culprit, and then you can more accurately begin a diagnostic and repair effort. If the people at AutoZone cannot help you to your satisfaction, you can take the p-code(s) here and any one of our Techs can tell you what it means, and give you advice on how to diagnose and repair the problem.