Hi
It could be a faulty temperature sensor in the freezer compartment.
Usually these are a NTC resistor (thermistor) type sensor. Its’ resistance value decreases as the temperature increases. I don’t know the specifications for the sensor in your freezer (model not stated) but if it measures as per its’ spec sheet i.e. correct resistance value at particular temperatures, then it may be a display board problem.
As can be seen in this refrigerator thermistor datasheet - example only Scroll down to Table.1 (not your sensor that I know of) the resistance changes from ~96K Ohms at -20 C (~ 0 F) to 32K Ohms at 0 C (32 F). You could always test the sensor by placing it in a glass of ice water and then measure its’ resistance and then leave it out and measure how quickly the resistance changes as it warms up to ambient temperature. Just a thought.
How rapid are the fluctuations shown by the thermometer?
Normally the freezer temp should stay reasonably stable at the set temperature or thereabouts once it has been reached, depending on how often and how long the door is open for of course.
Have you checked that the door seals are OK? With a lot of temp sensors they are mounted near the evaporator which is behind a panel and therefore not affected that quickly by temperature changes in the main compartment, due to possibly the door being opened at lot. Place a piece of paper between the door and the door jamb and with the door closed try to pull it out. It will come out with a bit of effort but not that easily and it definitely should not just fall out by itself. Try this at various places around the door, top, bottom and both sides
There are times that the freezer temp will increase to 32 F for a while and then go back down to the set temp (usually 0 F) which does take a little bit of time to achieve. This is when the refrigerator is in the auto defrost mode.
When the auto defrost mode is started, the compressor and the evaporator fan are turned off and the freezer temp is allowed to rise to ~32 F to melt the ice formed on the evaporator unit, which then drains away to the evaporator pan under the compartments near the compressor. The defrost heater is also turned on to speed up the melting process as you don’t want the food to even begin to thaw. Once the temperature reaches ~32 F the defrost thermostat operates to signal the control board to end the defrost cycle by restarting the compressor and the evaporator fan. The whole process takes about 15-25 minutes and occurs every 8-12 hours (depends on manufacturer).
The compressor and evaporator fan should only be off, if the set temperatures have been reached in both the refrigerator and the freezer compartments or it is in the defrost cycle. Otherwise they should both be operating trying to maintain the set temperatures.
Hopefully this may be of some help.
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