Date:         Thu, 6 Jun 1996 23:35:17 -0400
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         DHaynes57@aol.com
Subject:      Re: O2 sensor is a thermistor

In a message dated 96-05-29 21:10:13 EDT, carwile@ix.netcom.com writes:

>
>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 .-)
>
>

This is not correct at all.  The Sensor itself is actually a platinum coated
element.  When hot enough, the sensor creates a small voltage(yes, you can
actually measure it with a voltmeter) that is proportional to the ratio of
the quantity of oxygen in the outside air as compared to the oxygen content
of the exhaust gases.  If you more more O2 in the exhaust than
outside(impossible I know)  you would actually get a negative reading off the
sensor.  A thermistor as you implied changes resistance as a function of
temp.  The O2 sensor actually generates a small current.

Dennis.