Oxygen Sensors and junkyards

[LotusM50 at aol.com]

In a message dated 97-05-20 03:55:14 EDT, tboicey at brit.ca (Trevor Boicey)
writes:

<< Can somebody please provide me a brief rundown on the
 types of Oxygen Sensors? I forget the name of the device
 I am referring to, but I mean the simple type that can
 be used to give rough lambda readings and can provide
 feedback to closed loop systems.
 
   I know there are heated and non heated, and apparently
 they are known by the number of wires which each has. I
 suspect the "more wire" types are heated, but I want to be
 sure before I start spouting inferred facts. >>

On a similar note, the Electromotive system (no flames please) uses a 4 wire
heated oxygen sensor.  Does anyone know from what car Electromotive sources
this part from so I can get one at my local parts store.  I would prefer not
to pay Electromotive's price.  Also why 4 wires?  What does the 4th wire do?

-Bonn