Toyota Repair: 02 Sensor, air fuel ratio, fuel injection service

Question
Hi Ted:
   I have a 1990 camry 4 cylinder with 209,000 miles . I already changed the top 02 sensor. When I look near the manifold there is another sensor below the top sensor with 2 wires and a connector . Do I need to replace this too?  How often do I need to replace it?.  There is NO check engine light. Should I buy the universal 02 sensor or the direct fit one? Car gets bad gas mileage despite do a full tuneup fuel injection service, plugs, wires, air and fuel filter with 20 MPG on FWY and 18 MPG in town. It burns 1/2 quart of oil per 1000 miles. It runs good though. Can you help?
   Thanks in advance for your help

Answer
There are two sensors as you know, the one in front of the catalityc converter which reads the air/fuel ratio and the one behind the cat converter which does basically the same thing, it checks to make sure the catalityc converter is doing it's job. Unless the check engine light is on the system is functioning properly, you can replace it due to mileage/maintenance concerns. I would avice to use a plug in sensor and stay away from the real cheap ones, stay with a Denso if possible, they are available at most autoparts stores as well as online.