Why is the check engine light on for my 2009 Toyota Corolla?

A check engine light may be illuminated due to a vacuum or emissions leak which can be as simple as a loose gas cap. In the 2009 Toyota Corolla, more serious reasons include a faulty O2 sensor or a failing catalytic converter. T An O2 sensor often needs to be replaced if You suddenly notice a drop in fuel economy. It's responsible for monitoring the amount of unburned oxygen in the exhaust and conversely provides a key piece of data to the engine's combustion system in deterMINIng its air/fuELRatio. The cat helps transform carbon monoxide into carbon dioxide and can become clogged and d***y over time. When the ECU senses imbalanced emission readings, the "check engine" light will be illuminated.