I think there is a confusion between two different items here. There 
     are Exhaust Gas Temperature sensors which is what is being described 
     below.
     
     An O2 sensor works by a chemical reaction (working from memory here so 
     the details may be slightly off). They are actually comparing the 
     outside air with the amount of O2 in the exhaust gas. They have to be 
     heated up to about 400deg to start functioning. Single wire ones heat 
     from the exhaust itself so take some time before accurate. Multi-wire 
     ones have a built in 12v heater so they can start quicker.
     
     They are open to the outside air so they can compare, that is why they 
     can be destroyed by oil, etc. getting sprayed around the engine 
     compartment. 
     
     Jeff

______________________________ Reply Separator _________________________________
Subject: O2 sensor is a thermistor
Author:  carwile@ix.netcom.com at INTERNET
Date:    5/29/96 6:04 PM

The principle of the oxygen sensor in the exhaust stream is that of a 
thermistor which is yes, a heater.  A controlled amount of current is 
supplied to the thermistor and carefully measured on a continual basis. 
If the amount of current consumed by the thermistor decreases, then it 
is obvious that the thermistor is being subjected to heat from a source 
other than the supplied current.  In simple terms, the O2 sensor is 
measuring the exhaust gas temperature.  The computer compares the heat 
with a calculated factor based on rpm, throttle position, manifold 
pressure and all the other factors that determine how much fuel to burn. 
 Get the idea?   Lessie .-)