Your walk-in is drifting warm. Here is the order to check things in.

First, find out whether it is actually running warm

The wall thermometer is the least reliable number in the room. It reads the air at one point, usually near the door, usually at eye level, and it is often out of calibration by several degrees.

Probe the product instead, and probe it in two places: something on the top shelf and something on the bottom. That single comparison tells you more than any other free measurement you can take.

If the top is warm and the bottom is cold, the box is stratifying, which points at airflow. If both are warm and the unit is running, you have a capacity or refrigerant problem. If both are warm and the unit is not running at all, start with power and controls before anything else.

The ten-minute checks, in order

Work down this list before you call. Each of these is a real cause, and each of them gets found on service calls that did not need to happen.

• The door. Obvious and still the single most common cause. Check it latches on its own rather than resting closed, and check whether anyone has been propping it during deliveries. A walk-in door held open for a twenty minute delivery undoes several hours of work.
• The gasket. Close the door on a dollar bill and pull. If it slides out with no resistance, the seal is gone at that point. Do it at four or five places around the frame, including the hinge side, which is where gaskets flatten first.
• Airflow at the evaporator. Look at the unit inside the box. If boxes are stacked against it, or product is piled to the ceiling, the cold air cannot complete its circuit. There should be clear space in front of the coil and above the top shelf.
• The condenser coil. This is usually outside the box, on the roof or in a back corner. If it is packed with dust and kitchen grease it cannot reject heat, and the system loses capacity exactly when it is hottest. You can see this without tools.
• The thermostat setting. Worth ruling out. Settings get bumped, and new staff adjust things.
• What is in the box. If someone put hot product in without cooling it first, the box is doing a job it was never sized for. See the cooling section of our food code page for why this fails twice over.

Read the frost pattern before you call

This is the part worth learning, because it is free, it takes thirty seconds, and it narrows the problem more than anything else a non-technician can observe.

Look at the evaporator coil inside the box and at the insulated suction line leaving it.

• The whole coil is a solid block of ice. Almost always a defrost problem: a failed defrost heater, timer or termination sensor. The unit is making cold, and the ice is stopping air moving through the coil. This one gets worse quickly, because the more it ices the less air moves and the more it ices.
• Frost on part of the coil only, usually the inlet half. Points toward low refrigerant charge or a restriction, often a failing expansion valve. Low charge usually means a leak, and the leak matters more than the repair itself, for reasons in the cost page.
• No frost anywhere and the box is warm. Either the system is not running, or it has lost its charge entirely.
• Heavy frost or ice around the door frame. A failed perimeter or mullion heater. The gasket freezes to the jamb, the door stops sealing, and the box cannot hold. Cheap to fix, expensive to leave.
• Standing water or ice on the floor under the unit. A blocked condensate drain. This is a sanitation finding as well as a mechanical one, and an inspector will read it that way.

Running constantly and short cycling are different problems

Both get described as "it is acting up," and they point in opposite directions.

A unit that runs constantly and never satisfies is short of capacity. That is a low charge, a fouled condenser, a door or gasket letting warm air in continuously, or a box being asked to do more than it was sized for. In summer it can simply be ambient heat, especially on a rooftop condenser.

A unit that starts and stops every few minutes is cycling on a safety or a control rather than on temperature. Head pressure, low pressure cutout, an overloading compressor, a failing contactor. Short cycling destroys compressors, so it is worth treating as urgent even while the box is still holding temperature.

Say which one is happening when you call. It changes what the technician brings.

When it is the box and when it is the load

Refrigeration gets blamed for a lot of things that are really operational, and it is worth separating them honestly, because the fix is different and much cheaper.

A box that held fine in March and drifts in August is usually not broken. It is a unit that had adequate capacity at mild ambient and is short of it at summer ambient. A box that drifts on delivery days and holds the rest of the week has a door discipline problem. A box that drifts after the kitchen preps in bulk is being asked to pull down heat it was not built to pull down.

None of that means you do not need a technician. It means the conversation should be about capacity and operation instead of which part to replace, and a good contractor will have that conversation with you.

Protect the inventory while you wait

If the box is above 41°F and product is at risk, do these in parallel with calling.

• Keep the door shut. Every opening costs you. Decide what you need, get it in one trip.
• Start a temperature log now. Time and reading every 30 minutes, written down. If you carry equipment breakdown or spoilage coverage, this log is the first thing your carrier will ask for, and reconstructing it afterward from memory is not the same thing.
• Move the highest-risk product first. Raw proteins and dairy before produce. Another box, a reach-in, or ice in lidded containers will buy hours.
• Do not refreeze anything that has thawed. Check the date marking section of our food code page: a temperature excursion invalidates the seven day clock on ready-to-eat items regardless of what the label says.
• Photograph the thermometer and the product. Same reason as the log.

What to have ready when you call

Refrigeration gets quoted, never listed, and the first question is always diagnostic. Having these five things ready shortens the call and the visit.

• The equipment nameplate: manufacturer, model and serial. It is usually on the condensing unit, sometimes inside the box near the evaporator. Photograph it.
• Product temperature top and bottom, and the air temperature the thermometer claims.
• What the frost looks like, in the terms above.
• Whether it runs constantly or cycles, and roughly how often.
• When it started, and what changed around then. A delivery, a power event, a cleaning, new staff, the first genuinely hot week.