Frigidaire Freezer Model FFU20F9FW1 defrost...

Question

Frigidaire Freezer Model FFU20F9FW1 defrost problems

Recently had a total defrost of all items in freezer. Replaced defrost timer and cleaned everything plugged in and the unit ran for a little over 2 weeks no problems. Today went out to put something in the freezer and found it 40 degrees inside once again. Called local parts place and was told that a limiter - thermostat could be the problem. Any help or suggestions? Some what mechanically inclined but instructions a six year old could follow would be a help. Thank you