After checking the defrost heater to confirm it was ok. I replaced the main electronic board. That wasn't it. After 1 1/2 weeks, it frosted up again. I called and talked to a tech person at APP and then ordered the air tower, defroster thermostat, and temp sensor. I installed them all and the frig works great. I know replacing all those parts at once didn't confirm which one failed, but I just wanted it fixed. The cost was less than having a service call. The parts arrived the day after ordering them. Thanks, Appliance Parts Pros!