Toyota Repair: Oxygen Sensor and Air/Fuel Sensor, oxygen sensors, haynes manual

Question
I have 1998 Toyota Avalon,Lately I'm getting less mileage after fuel induction service and new spark plugs(a year back),I decided to replace Oxygen sensors on my own even there is no OBD code.The haynes manual says there are 3 oxygen sensors(2 before catalytic and one after catalytic),I was wondering the one after catalytic

Answer
Unless there is a check engine light on all sensors are working normally, the one behind the cat monitors the car converter operation, if the converter is not working efficiently the sensor will pick this up and turn the check engine light on, this sensor does not control anything, all it does is read the oxygen content of the exhaust gases.