A question about default mode vs. adaptive memory
 

My truck is a 2003 4.6L SuperCrew with 150K miles.

What events or scenarios will cause the computer to stop using its adaptive memory mode (which it learned from my personal driving habits) and switch to its built-in default mode?

And what must be done to get it to start using adaptive memory mode again after this switch occurs?

The reason I ask is because I recently started getting very poor gas mileage (compared to normal) and the only thing I can think of that might have caused this change is that the computer switched from adaptive memory mode to default mode. Does this switch occur when the check engine light comes on, or when it starts flashing? Because both of these have happened relatively recently.

I drive conservatively so I think the truck should get better gas mileage using adaptive memory than default mode. Am I wrong?

By the way, are there any other driving modes in this truck, aside from fail-safe (limp-home)?

You sort of got the two mixed up.
Adaptive does not actually stop under any conditions.
Poor drivability or fuel milage has little to do with adaptive learning.
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Adaptive learning is for cold starts, hot restarts, load changes from power steering and A/C loads changes etc
In program live memory, fuel trims changes are saved long term and set code when limit is attained etc.

Poor mileage is investigated by looking at live and saved data over time for a reason to be to close or out of normal limits as causes for the observations. Vacuum leaks causing fuel shifts in the rich direction as the system tries to compensate for the leak etc. This is were you see the history and the point limit is attained and code set.
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You can see this has little to do with adaptive learning for basic normal functions of cold starts, hot starts and load corrections etc.
To mention, closed loop fuel control can be lost and result in poor fuel milage but still not affect drivability very much except in the wallet. When this happens, the program substitutes a fixed value from fuel tables so the vehicle can still be driven and not disable for no good reason.
Good luck.

Yes, this sounds like what I read about somewhere else, I just didn't know what it was called. Apparently this is what has happened to cause my gas mileage to plummet ... so how do I get my "closed loop fuel control" back to the way it was before?

IF..that is what it going on, there should be code set.
A look at live data with a Scanner to see what the conditions are should suggest what to do for the repair.
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Are the long-term fuel trims shifted to rich?
Are the Ox Sensors switching?
Are the Ox Sensor Heaters working?
Is there an intake tract leak etc.
These items have to be looked at before the possible causes are considered.
Good luck.