The O2 sensor can only give the DME a certain range input to make adjustments so much. If the AFM is so far out of range, the O2 won't be able to help. Its like shooting a rifle adjusting scope when the target is nowhere near the field of vision of the scope. Only when you get the target into the field of vision by adjustment of the AFM will the fine adjustment of the scope (O2 sensor input to the DME) make any difference at all.

 

Legoland951, the light bulb just went on over my head. I take your point, and that seems to be the most likely explanation. Thanks for the input.

I'll try swapping the unit with a known good used one to see where that gets me.

Or I may take this opportunity to attempt the MAF conversion I've been contemplating.