Check Engine Lite is on on 2002 GMC Sonoma

What are the possible problems associated with the illumination of the Check Engine Lite.

Asked by for the 2002 GMC Sonoma
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Go to a Advance or Autozone parts house and ask them to code out the computer. Ask for the code numbers. e-mail me at
Can't help without a TROUBLE CODE
02 sensor front of catalytic converter or rear of converter.