A yellow light came on saying Service engine soon. What does this mean?

Everyone that commented do have valid points, don't replace parts based on trouble codes alone and have a competent person actually diagnose what is wrong!It will save you money and frustration in the long run. Nissan/Mitsubishi/Chrysler all say Service Engine soon which is misleading it doesn't require routine service as you would be led to believe by that message. If the Check Engine Light/Service Engine Soon light illuminates while you are driving, the car's on board computer sees a fault in the engines management system that is causing an excess amount of pollutants to be emitted from your car. This could result in engine damage if left neglected. Some problems that can cause the Check Engine light to stay on may take two "drive cycles' to complete before illuminating the Check Engine Light. The computer sees a problem and looks for an identical set of circumstances to occur to confirm that it has indeed seen a problem before turning on the light. A Check Engine light that continually flashes while you are driving is serious and is a sign that the computer sees a problem that seriously risks damaging the catalytic converter which may result in an expensive repair or even may cause a fire! Diagnostic codes are stored in the vehicle's computer and can be retrieved using a scan tool that communicates with the computer in your car. We have in our Repairpal website an encyclopedia with in depth articles about computer, and computer control systems, emission and driveability problems. http://repairpal.com/check-engine-light

YOU HAVE A PROBLEM WITH A SENSOR,HAVE CAR SCANNED AT AUTO ZONE TO SEE WHAT CODE IT HAS THEN POST THE CODE. THEN WE CAN HELP.

I took one of my cars to Auto Zone to have the codes pulled. The guy told me the code was that my starter was bad, so assuming he was right I spent the $ on a new starter only to find out the car still had the problem once I had the starter installed. I took it to another local mechanic and he pulled the codes and told me it just needed a new battery. Put a battery in it and ran fine...Point here is I suggest going to somebody you can actually trust to tell you the truth! some places will tell you anything and chances are since youre in their parking lot they know youre probably going to buy the part from them on the spot!