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Didn't you mention you had 1.5 volts on the sensor lead? If you disconnect the sensor and measure again, the ECU should not provide that much voltage, which is really the offset adjustment for the high impedance amplifier that measures the output of the sensor. It should be right near 1/2 volt, maybe a little less depending on the input impedance of your meter.
Yes, it is nice to have a new sensor to have confidence. You can get one for $17 if you don't mind splicing the wire, or $25 if you want to plug it right in.
But, you might be something like curious to the point of making certain you are replacing the correct parts before ordering them. The computer should be pretty common in the junkyards, and your model is particularly likely to be good in a junker. And I follow your logic suspecting your ECU if sending 1.5V to the sensor and a dubious survivor of a battery to ground fault that burned up the chassis-to-valve-cover ground strap.
(Edit: I see that was Dave Barton's ECU that had the offset problem at 1.5V.)
As for the fluctuation of the sensor's voltage, that will not occur unless the feedback mechanism works, i.e. the frequency valve is responding to change the mixture the opposite of what it sees at the sensor. That "Catch-22" is probably why the cheapest and easiest part is replaced first.
By the way, my k-jet car is pre-lambda, or more correctly not California when they were - so I do not have the benefit of being able to set the mixture using the sensor's output or the duty cycle of the frequency valve. Your system is better, I think.
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Art Benstein near Baltimore
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