Is the freezer evaporator fan running right now? Have you noticed decreased cooling in the refrigerator section because of this problem?
I'm attaching your tech. data sheet below. Enter into the diagnostics and do TEST 6.
Service Test - 6 Defrost heater/Bi-metal
Note: if bi-metal is open, it will need to be by-passed for heater to operate. Heater should be on. Display will be blank until a valid reading is displayed. (01 = Bi-metal closed, 02 = Bi-metal open)
The Bi-metal is the defrost thermostat, it needs to be frosty for the defrost heater to come on. If you haven't defrosted it then it should be covered in frost and the defrost heater should come on. Feel for heat the bottom vent on the back panel in the freezer.