I am pretty sure that your Corolla has two O2 sensors. The O2 sensor located after the catalytic converter (known as sensor #2) does not have any effect on fuel economy or engine performance. The purpose of sensor #2 is to monitor O2 in the exhaust after it has passed through the catalytic converter and compare it with the O2 that sensor #1 is monitoring. If they are the same, then the catalytic converter is not working properly and the engine computer illuminates a check engine light.