Check engine light came on. What are some possible reasons for this?

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97 Toyota Avalon (V6) with 162,000 miles. However, the engine was rebuilt (due to a recall) by Toyota back at 89K. The Check engine light first came on last week. I tried to do a reset and it worked for about a day and then the light came back on. I realize I will need to have it looked at but I was hoping for a little heads up on the possible causes for the light being on. Thanks, Joe