First you have to know what type of control the cooler has. If it is a larger cooler, it will be suction control, not a thermostat. The thermostat inside will be a back up for the user. a refrigeration tech would have set the suction control with a calibrated guage so that the cooler evaporator gets to 17 -19 degrees before shutting down the compressor and let’s the coil come up to 38 degrees before coming on. This prevents icing and provides maximum efficiency. If the cooler suddenly starts to freeze product, either it hasn’t had a normal load for a while or is low on refrigerant. Check the sight glass for bubbles after the compressor starts and runs for about 5 minuets. Also it is common for certain areas of a cooler to be at 31 or 32 degrees F for a bit Usually low on the opposite wall of the evaporator fans. Endive and other fragile produce needs to be kept high and to the sides of the evaporator. (warmest area). This is something that a refrigeration tech knows and you will need one to charge the unit properly. let alone find the slow leak.
I should add here that there is a very large percentage of people working on walk-in’s that do not understand the importance of using suction control or have the gauges for successful adjustment. Back when all that was available were analog gauges one had to have a new gauge that was never used except for this adjustment. If over pressured once, delegate it to your manifold and buy a new one. With the new digital manifolds that also have temp sensors, it’s far easier. No more carrying pressure charts and you have a direct read-out of the evaporator temp. Eliminates all the call backs for icing or warm box from ambient condenser temperature variations. We get wide swings in the northeast. Sometimes 70 degrees F in 12 hours. And if the condenser doesn’t have the right accessories it becomes a real problem. Those that don’t fully understand what is happening get frustrated and migrate to a temp control and adjust the suction control wrong . This either makes for icing or shortens compressor life from slugging.
Head pressure control, crankcase heaters, and suction accumulators are mandatory here with outside condensers