Rear O2 Sensor, what is it for?

If the front O2 sensor effects your gas mileage, what does the rear one do? I haven't read any threads on here where someone said you should replace your rear O2 sensor.

Thanks

If the front O2 sensor effects your gas mileage, what does the rear one do? I haven't read any threads on here where someone said you should replace your rear O2 sensor.

Thanks

The rear o2 sensor is just for monitoring the cat and does not effect mileage,unless the cat is bad.


~Larry~