I think they say a bit about it in the video posted above, but the point of calibration is to make up for variations in the resistance of the sensor assembly caused by variations in material and the manufacturing process. The AEM sensor includes a resistor that is tailored to the sensor so that, unless the sensor is broken, the output of the assembly is exactly the same for every one that they build. That means they can make the gauges all the same and they don't need to allow for variance in the sensors. Also, calibration is unneeded. The sensors are internally heated to a specific temperature, so that is not a factor.
In order to tailor the resistance, they have a resistor that is basically a strip of material. As they measure the sensor output, they chop off some of the resistor until the output matches their spec exactly. So each one is individually tailored.
Basically, either your sensor reads correctly or it's broken / fouled.
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