checking oxygen sensor output voltage

[Rustam]

Today I experimented with oxygen sensor with output disconnected from the computer but connected to a multimeter. Before attempting this I was convinced that I would see a steady value of voltage output since the sensor is not connected to the computer. At first that is exactly what shows, but then the voltage starts dropping and settles at .01V.

Has anyone ever done this? The symptom is such as would have been if the connection to the heater element was terminated - but the jumpers were there and in fine condition...

Any comments?

[mikell]

Check out Bruno's site - bmwe34.net - there is a decent write up on O2 sensor testing.

[Bill R.]

disconnected you have to have a warmed up engine then disconnect and run it at 2000 rpm minimum for at least 2 minutes. Then when you let it idle you'll get a reading but if you run it at idle long the sensor will cool down quickly and the voltage will fall off to nothing.



Today I experimented with oxygen sensor with output disconnected from the computer but connected to a multimeter. Before attempting this I was convinced that I would see a steady value of voltage output since the sensor is not connected to the computer. At first that is exactly what shows, but then the voltage starts dropping and settles at .01V.

Has anyone ever done this? The symptom is such as would have been if the connection to the heater element was terminated - but the jumpers were there and in fine condition...

Any comments?

[Rustam]

disconnected you have to have a warmed up engine then disconnect and run it at 2000 rpm minimum for at least 2 minutes. Then when you let it idle you'll get a reading but if you run it at idle long the sensor will cool down quickly and the voltage will fall off to nothing.

Bill, I connected jumper wires to provide the power to oxygen sensor from the oxygen sensor connector at the end of the computer wiring harness...
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The reason why I saw such low voltage was that the AFM was misadjusted towards very lean mixture.

Today I tried to turn the adjustment nut on the AFM to see a response and I did see one. Turning clockwise provided the expected "rich mixture" voltage ~.85V. What puzzles me now is that this voltage is very steady, but when the nut is turned counter-clockwise to make the mixture lean the voltage starts fluctuating very haphazardly.

Any ideas what may contribute to the fluctuation when the mixture is leaned?