First question: everywhere, or one spot?
This single distinction decides whether you have an equipment fault or a storage habit. Take readings in three places: directly in front of the evaporator, in the middle of the box, and in the far corner away from the coil. Five minutes of work that saves a wasted service call.
If the whole box is too cold
The system is not being told to stop. Three causes, in the order of frequency we see them.
Contactor contacts welded shut
The most common and the most urgent. Contactor contacts arc every time they open and close; eventually they fuse. The control circuit says stop, and the compressor keeps running because the power path never actually broke. A box that runs continuously will ice its evaporator solid within a day or two, and then you have a second, worse problem on top of the first.
Thermostat or controller failed
Mechanical thermostats fail closed as the bellows loses charge. Electronic controllers fail less often but their relays weld in exactly the same way the contactor does.
Sensor drifted or in the wrong air
A sensor reading warmer than the box really is will hold the system on long past setpoint. This is the quiet one — nothing is broken, the control is simply working from a bad number. Sensors drift with age, and a sensor sitting in the return air path near a door reads warm every time someone walks through.
If only part of the box freezes
The equipment is working correctly and the layout is wrong. The air leaving a walk-in cooler evaporator is 10°F to 12°F colder than the box setpoint — so a box holding 36°F is throwing air at roughly 25°F out of the coil. Anything sitting directly in that stream freezes, and high-water-content produce freezes fastest.
The fixes are all free:
- Keep an 18-inch clear zone in front of the evaporator and never shelve into the discharge stream.
- Move lettuce, herbs, cut produce, eggs and bagged salad to the far end of the box.
- Do not block the return air path underneath or behind the unit either — starving the return makes the discharge colder and the problem worse.
- If the box genuinely has nowhere else to put product, an air deflector on the coil redirects the stream. That is a real fix and a cheap one.
The cost of ignoring it
Frozen and thawed produce is a total loss, and it is a loss that repeats every day until someone deals with it. Dairy that has frozen separates. Eggs that freeze crack and become a contamination issue rather than a quality one. Operators tend to absorb this as shrinkage for months before mentioning it to anybody, which makes it one of the more expensive cheap faults in a kitchen.
What we check
We verify the control is actually opening the circuit, test the contactor under load rather than just looking at it, check sensor accuracy against a calibrated reference instead of trusting the display, and map the air pattern in the box so the storage layout matches where the cold actually goes. If it turns out to be a layout problem we will tell you that and charge you a diagnostic rather than inventing a part to sell.
Questions we get asked
Only the top shelf by the coil freezes. Is the cooler broken?
No. That is the discharge airstream, which leaves the evaporator several degrees below box temperature by design. Nothing with high water content should sit in it. Leave an 18-inch gap in front of the coil and the problem disappears.
What temperature should a walk-in cooler actually run?
35F to 38F. That gives headroom below the 41F Massachusetts limit for normal defrost rises and door openings without taking product near freezing.
The box will not stop running at all. Is that dangerous?
For the compressor, yes. A system that never cycles off runs its coil below freezing continuously, builds ice, and loses the ability to defrost it. Switch it off at the thermostat and see whether the compressor stops. If it keeps running, the contactor has welded and it needs attention the same day.