The refrigerator was given to me and hasn't defrosted since it was new. I found the defrost timer had a burnt spot on the circuit board. After installing a new timer, the refrigerator would trip the circuit breaker in my house when it entered defrost mode. I found that the pink wire running from the control board all the way to the thermostat was shorted somewhere between the two parts, inside the refrigerator. I ran a new "pink" wire and all is good.