The ONLY reason the light comes on is to inform you that there is a stored engine diagnostic code in the computer and that you should use a code scanner to figure out what's wrong. Quite often the problem is simple, like an oxygen sensor. If you read the code and it IS an oxygen sensor, replace both sides. Usually the rear sensors don't give much trouble. If the idea of reading the diagnostic codes is a little overwhelming, go to an auto parts retailer and ask for a little advice. They're usually willing to help connect the code scanner and read the codes with you if they aren't too busy. In the West we have Pep Boys, Auto Zone and Checker Auto that are all usually willing to help and sell parts at reasonable prices, but Pep Boys often try to force you to let them do the work afterward.