This is probably a long shot, but do the symptoms below make it obvious to anyone what's wrong with my HEVAC (MY2000 4.0 P38).

After 20 seconds to 5 minutes of driving, the book error comes on.
Temp control for drivers side changes number, but has no impact on the temp coming out of the drivers vents.
If I leave it on a cold temperature however, when I restart the car, I get cold air.
But then the book comes on and I'm stuck with cold air.
Same if I leave it on a hot setting.

I know without a Nanocom or similar, there's a whole bunch of things it could be, but does anybody reading this think it's clear what the issue is?