It stands for "Service Engine Soon"
The light will come on when one of the sensors in the vehicle sends a signal to the computer that something is out of range.
If you take the vehicle to autozone, they will hook up a scan tool for free and give the the codes that will have been stored in the computer for diagnosing the problem.
Once they give you the codes, you can list them here and we will look them up for you and help to steer you in the right direction to get it repaired.
If you have further questions on this, please feel free to reply.
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What year is it?