P0420 code has me stumped

I'll keep trying to back out the set screw, but I don't know that I have much further to go. Right now if I unplug the IAC it will idle for about 5 seconds before dying, which is better than before when it would die right away.

5-sec sounds decent. But you're backing the screw out? If you're backing it out you're closing the throttle plate...
 
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