Re: Oxygen Sensor Question

["Scott Mo." <scottmo@teleport.com>]

>Mark asked:
>Does anyone know what the output from the o2 sensor should be.  I know it
>should generate a .5 volt signal at lambda, but the reading I am getting
>from mine fluctuates from about .2 volts to 1 volt.  It just swings back and
>forth erradacly.  Is this normal or should it be more of a steady reading?

What you describe sounds like normal O2 sensor operation, assuming the
voltage fluctuation is occurring quickly enough at idle. The O2 sensors
tend to respond more slowly as they get older and contaminated. Usually the
O2 sensor voltage will complete a high to low, low to high transition once
per second at idle, 2-5 times per second under cruise conditions. 

The O2 sensor  is responding to the slight tweaking of the fuel mixture by
the ECU. The mixture is slightly rich (less O2), then the O2 voltage swings
high, the ECU leans out the mixture (more O2), then the O2 sensor voltage
swings low, and the ECU richens up the mixture, the O2 sensor voltage
swings high again. Over and over, closed loop feedback mixture control.

If the O2 sensor voltage is stuck high or low, it usually indicates a
problem with the engine/ECU/fuel system and not with the O2 sensor itself.

O2 sensor problems usually show up as surging during cruise or during warm
up after the system goes into closed loop. It is a good idea to replace
them every 75k miles.

I have a picture showing the electrical waveform from the O2 sensor on my
web page. This was taken using an Oscilloscope with the engine idling.

http://www.sjmautotechnik.com/ecusys2.html#o2

HTH
Scott Mockry
scottmo@teleport.com