The LED adjustment procedure presumes everything affecting mixture is in the ball park to begin with. Stuff like vacuum leaks, fuel pressure, injectors will throw the mixture off more than that pot will correct. If the O2 sensor isn't working, e.g. clogged with soot from rich running, the LED test point will not switch. It is actually monitoring the output of the first circuit inside the ECU that handles the O2 sensor signal, so the flashing indicates the switching between lean and rich that occurs while the ECU is maintaining the correct mixture. Because AMMs are slightly different in response, an adjustment is given on the AMM to adjust the ECU to match it up until LH2.4 when the ECU became automatically adaptable to the slight variations in sensor output.

A better procedure to use in replacing AMMs would have you measuring the voltage on the yellow lead (LH2.2) before you adjust or disconnect the old one. Then you could at least put the pot back where it was if needed or set the new one to the same value before installing it. You could measure the resistance, but it is much easier to backprobe the voltage as you set the pot-- it varies from zero to about 2.7V-- then you don't need to guess where you ran out of adjustment. The ends are very hard to feel.

Check the voltage at the oxygen sensor; you'll probably find it is not oscillating between .1 and .9V. It must do this for the LED to flash. If not, it doesn't definitely mean the sensor is not working; it could just be reporting what it actually sees, either too rich or too lean.
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Art Benstein near Baltimore