Toyota Repair: Service Engine and Trac off Lights, vsc control, toyota avalon

Question
I have a 1999 Toyota Avalon XLS. My "Service Engine" light came on and shortly thereafter the "Trac/Off" light also came on.  Toyota states it is the oxygen sensor on the exhaust manifold, #1, on the front.  The mechanic at the garagbe is stating it is the "Traction Control Motor".  The scan done by Auto Zone states it is the oxygen sensor also.  
Who is correct?  Is there a way to be absolutely positive? Any idea of "cost" to repair or replace what is needed? Time involved?
thanks in advance!  

Answer
Whenever the check engine light comes on it will disable traction and VSC control, the root cause is the check engine light so the O2 sensor problem should be addressed first because there is probably nothing wrong with the TRAC control. Since this engine has two O2 sensors, one for each bank I would need to know the trouble code number to tell you which side needs to be replaced.