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When the Service Engine (aka Check Engine) 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.
You can reset this light yourself by removing the "NEG" cable from the battery for 15 minutes, however, be aware that if the problem that is setting the light is not correctly repaired, the light WILL return within 50-75 miles of normal driving.