The freezer coils were icing up, so I replaced the defrost heater and thermostat. Defrost cycle now working fine, and the freezer temp went from stubbornly stuck on +22 degrees to -10. I turned the freezer temp control down and heard the damper close. Freezer went back to +20 and hasn't gone lower for 2 days, even with temp control back at max, even though the freezer coils are defrosting regularly.
Seems logical that the thermostat that controls the freezer damper might be bad, but I'm wary of the coincidence that the defrost heater, defrost thermostat, and fridge thermostat are all bad.
HELP... I must be missing something here. I'm not an appliance tech, but I'm a pretty good parts replacer. Having followed advice from other sources and tested/replaced the defrost system parts and still have the problem of a too warm freezer, does anyone have insght into what the problem is (before I end up dismantling this refirigerator peice by piece as I replace one part at a time)?

Thanks for the quick reply!
Freezer evaporator fan is running. I assume the damper has not opened up; I didn't hear it move when I changed the freezer temp control back to max setting.
On the "Liner Parts" diagram, the left knob #22 controls the thermostat #13. In my fridge this is labelled as the fridge temp, while the right knob #22 is for freezer temp. So mine are either reversed from those shown in the diagram - or the damper controls how much cold air flows from rather than to the freezer. Does this sound right or am I not seeing something?

Then the damper seems to be stuck in the open position, letting too much of the cold air out of the freezer? What would make it stay open... bad damper or knob? Bad thermostat? What should I check next? Thanks for your help.