Heavy Frost on Evaporator
First, always check door ajar, customer usage - numerous door openings, etc...
Unplug refrigerator from power. Unplug blue connector from main board. Measure between blue wire on connector and neutral orange wire on main board J7, pin 9.
Is there approximately 22Ω ?
No
Check wiring harness Check defrost heater
Check defros safety thermostat
Yes
Verify thermistors are within proper range using temperature resistance chart on page 49
Is the resistance within range?
No
Check wiring connections
If wiring is OK,
replace thermistor
Yes Replace 
main board
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