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David,
I'm also unsure if it is an accurate conclusion to disregard the AMM. I think it may be an accurate first step, and economically sound decision. Then you may later conclude the oxygen sensor outlasted the AMM - very possible.
I was going to try to describe the output curve of our oxygen sensors to dissuade you from using its static output voltage in the way that you are. But I found David Hunter's comments to be right to the point: if it is in closed loop, the AMM is in range by definition. You have to see this at idle and under load. That pot on the AMM is a very subtle adjustment; if you can equalize the duty cycle with it, you're doing better than most folks.
The oxygen sensor's output waveform is judged by the rise time on lean/rich transition. Contamination by oil fouling, carbon from a rich mixture or, like I had once, tranny fluid on the outside reference port will affect the volume of oxygen passing through its substrate. With a given exhaust pressure, a reduction in volume would seem to lengthen the reaction time to mixture changes, analogous to you building a bookcase around your home's thermostat. Add to that the effect of contamination on the chemical makeup of the solid electrolyte. There are some good waveform pictures and examples on the web, I think you'll find by searching for the term "lazy sensor", "Nernst cell", or such language.
Also, if you haven't, check the heater by measuring its change in resistance immediately after unplugging.
You may be right on track with the AMM for cruising, because it may be squeaking into the closed loop condition only at idle or no load. I think I have an -007 with better output I'd be happy to send you to try. But the only indication of mixture to the system comes from the transitions of the narrowband oxygen sensor.
If you have a wiring diagram, and you will be in the cluster anyway to replace your gear, you might consider running that mixture test signal from the ECU to one of the unused postive referenced idiot lamps in the cluster, like upshift if you have an auto or EGR if you don't have, so you can watch the mixture reactions to driving. The LH2.2 has a good sized transistor buffering the output, so it will handle a lamp as well as an LED, just be sure to keep a handy way to disable it or the blinking light will become annoying.
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Art Benstein near Baltimore
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