My chest freezer decided in the last few weeks to stop working well for me. I would come out and see if sitting around 15-20F. I turned it off and back on a few times and each time it would work for a day or two then get stuck at 15-20F again. So a few days ago I moved all the food out and into another freezer and put an external temp controller on the freezer. I set it at -5F and it is now holding no problem.

I noticed before the freezer would just run and run and the back wall inside the freezer would get very warm (in the 20's or 30's) while the front inside would be -25F. It seemed to be fighting itself. Well now with the external temp controller both walls are nice and cold.

Is this something with the thermostat or a sealed system issue?

This is the external thermostat I have been using.

Series TS | Digital Temperature Switch | Dwyer Instruments