Step One: Checked Appliance Parts Pros.com for video of symptom and repair process.
I blocked vent between freezer and refrigerator by inserting a plastic grocery bag from the freezer side (behind the ice bin)to block the airflow, then confirmed temperature rose in refrigerator. This confirmed the damper assembly was not working. (the fan was working, but after removal I found the small door had broken off the hinge).
Replacing the Damper Assembly fixed the problem.