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O2 sensors. Was "Why measure CO?"



After doing a lot of research about O2 sensors in preparation for installing
an aftermarket engine management system, I've found out the following.
The output of any average 1,2, 3, or 4 wire sensor is accurate only from
~14.2 to ~15.2. In other words, the voltage is essentially 1v at 15.2 and 0v
at 14.2. By design the sensor has a very steep transition from 1 to 0v
either side of stoichiometric.
Heating the sensor above 600 deg C tends to flatten out the curve on the
rich side giving it the ability to measure A/F down to about 12.5.
Unfortunately, you need to know how hot the sensor is to know what voltage
corresponds to what A/F ratio. So, essentially it needs a control for the
sensor heater and it needs a method of measuring sensor temp.
The third alternative is the five wire sensor. This is actually two sensors
in one body. One surface exposed to air, one to exhaust and an intermediate
chamber between the two. A current is supplied across the air/intermediate
chamber interface biasing it so its O2 content approximates that in the
exhaust. In short this linearizes the sensor so that voltage applied to the
sensor "pumping chamber" is inversely proportional to A/F ratio from 10:1 to
20:1 with an accuracy of .1 A/F. This also needs a special controller.

www.fjoinc.com has some interesting reading

The Honda Insight is the only car I know of that uses the 5 wire sensor.

I guess my point is, trying to use a standard O2 sensor to read A/F below
14.2 is a waste of time.
Dan
As usual, sorry for the long post.


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