Lean Condition

You guys seem to be up on most everything about these cars and I appreciate the ideas I have read about other people's problems. Here is my situation: 1996 Aurora with a po171 174 lean condition. I have an Autoxray scanner and I can't seem to get any response from o2 #3. I guessed that it was the post cat sensor and replaced. After running 02 test again #3 still had no data. I then disconnected both pre and post sensors to see if I had replaced the wrong one and only o2 #3 still had no response. There are no other codes except lean bank one and two. Any Ideas on what I am doing wrong with this scanner or if I guessed the sensor location wrong?
Any input would be most appreciated. Thanks in advance!!

Try replacing it was acdelco- i know bosh does not get along with GM. This is my experience. it happened to my grand prix and my alero. it may be your problem. please try that. just a thought.

Thanks VAMC will try. I checked MAF readings with specs and they were low. I Cleaned the MAf- still low-replaced the MAF-still low. I just want to get this car to pass emmissions!!!!!

Problem fixed! The cause was a bad valve on the intake manifold. The gasket had pulled away from the valve.