Quick answer: A walk-in cooler freezes food when the box drops below 32°F, and the usual causes are a thermostat set too low or failing, or product stacked directly in front of the evaporator fan. A cooler should hold 35 to 38°F, not freeze.
Frozen lettuce or cracked eggs mean a walk-in is dropping below 32°F when it should be holding food just above freezing. Most of the time the cause is the temperature control or where the product is stacked, so check those two before assuming a failed part.
Check the setpoint and the control first
The simplest cause is a thermostat set too cold. A cooler should sit at 35 to 38°F, the range that holds product under the FDA Food Code limit of 41°F without freezing it, so confirm the dial is in that range first. Then watch whether the compressor cycles off when the box reaches setpoint. If the control never satisfies, or a thermometer reads 30°F while the dial says 38°F, the temperature control or its sensor has drifted and is letting the box overcool. See what temperature a walk-in cooler should be for the target and the buffer behind it.
The cold spot in front of the blower
The air leaving the evaporator fan is the coldest in the box, often several degrees below the setpoint, and product parked in that stream freezes while the rest of the cooler reads fine. Keep boxes and pans out of the direct discharge, leave a few inches around the coil for air to move, and never stack product against the evaporator. When only some of the food is frozen and the rest is fine, this is the first place to look.
When the whole box is freezing
If everything is freezing, not just the food near the fan, the refrigeration is not shutting off. A stuck contactor or relay can hold the compressor on so it runs nonstop, and a failed temperature sensor can leave the control blind to how cold the box already is. Both drive a cooler well past freezing, and both need a technician, because they are electrical faults rather than a dial you can turn.
What you can fix, and what needs a tech
You can reset the dial to 35 to 38°F and move product out of the fan’s path in a few minutes. What you cannot fix from the floor is a miscalibrated control, a stuck contactor, or a sensor reading the wrong temperature. If the box keeps freezing after the setpoint is right and the product is clear of the coil, the controls need a look before a full cooler of product is lost.
Most coolers that freeze product come back with a small control or sensor fix. ProToCall handles it before the next delivery freezes. Call 866-297-5082 or book commercial refrigeration repair.
For the opposite problem on a smaller unit, see why a reach-in refrigerator is not cooling.