Ford Repair: o2 sensor, ford taurus gl, 1996 ford taurus gl

Question
1996 ford taurus gl 3.0 soc engine. cat converter has two sensors one in front, one in back, my o2 needs to be replaced, told front is o2, what is back one? thanks bill

Answer
William, most engines today use 4 sensors to detect the oxygen content of the exhaust stream (thereby determining if the fuel is being burned cleanly)... each bank will have one in the exhaust manifold, and another after the catalytic converter.  The 'upstream' sensors are known as O2S11 and O2S21, and the 'downstream' sensors - or 'cat monitors' are known as O2S21 and O2S22.  
Now, if you're reading a scan tool you can watch the switching rates of the sensors... the ones that are upstream (in the exhuast manifolds) should be switching from low to high voltage rapidly, and the ones that are after the cat converter should be nearly level about midrange.  If they are switching rapidly, it doesn't mean the sensor is bad, it means the cat converter isn't cleaning the exhaust and needs to be replaced.  If they are always low, it could be a lean condition due to an exhaust leak allowing air into the stream, or it could be a dead sensor.  If they are high, then an engine missfire may be causing combustion in the converter of unburned fuel.
A trained technician at Ford has a computer to watch the sensor outputs on a graph scale... that is the best way to know whether sensors are working properly.
Hope this helps...
Clay