If frost is building up in the freezer, check first for a break in the door gasket that could be letting in warm, moist air. If the gasket seems sound, a malfunction somewhere in the automatic defrost system could be the culprit. A failed defrost timer or a bad electronic control board may not initiate the automatic defrost cycle, causing excessive frost buildup on the back wall inside the freezer. A faulty defrost sensor can prevent the defrost heater from turning on the defrost cycle. See how to check the defrost sensor and other components in the defrost system in our refrigerator won't cool troubleshooting video: searspartsdirect.com/diy/article/refrigerator-wont-cool-troubleshooting-video-defrost-system-problems
Refrigerator: Frost buildup in freezer repair guide: searspartsdirect.com/diy/symptom/refrigerator-repair/1234589/frost-buildup-in-freezer/ref0003
Enter your full model number on www.SearsPartsDirect.com
for parts and diagrams. If your model is the one tagged, here are all models beginning with 111.71212: searspartsdirect.com/search?q=111.71212
You can call Sears Home Services at 1-800-4-MY-HOME for repair.
Hope this helps!