1. Doors, gaskets, and traffic
Start with the cheapest cause. Check that the door closes fully and latches, that the closer still pulls it shut, and that the gasket is intact rather than torn, flattened, or iced.
A quick test: close the door on a dollar bill and pull. If it slides out with no resistance, the gasket is not sealing there. During a busy service, a door propped open during a delivery will raise box temperature faster than any mechanical fault.
2. The evaporator inside the box
Look at the coil in the ceiling unit. Light frost is normal. A coil encased in solid ice is not — iced coils block airflow, and the box warms even though the equipment appears to be running.
Heavy icing usually points at a defrost problem: a failed defrost timer or board, a bad termination sensor, or failed defrost heaters. It can also come from a door seal letting humid kitchen air in continuously. Either way, the ice is the symptom.
3. The condenser outside the box
Find the condensing unit — on the roof, out back, or above the box — and look at the coil. In a commercial kitchen these pack with grease, dust, and lint, and a blocked condenser cannot reject heat.
Confirm the condenser fan is actually running. A condenser fan that has failed will cause the system to run continuously while losing ground, and will eventually trip the compressor on high pressure.
4. Load and airflow inside
Check whether product is stacked against the evaporator or blocking the fan discharge. Cold air has to circulate; boxes stacked to the ceiling in front of the coil create warm pockets and skew your thermometer reading.
Also check whether hot product went in recently. Putting warm stock into a cooler raises box temperature substantially and takes hours to recover.
When to stop checking and call
Call immediately if the box is above 41°F and climbing, if the compressor is short cycling or will not start, if the coil is iced solid, or if you are hearing the compressor try and fail to start.
Move product to a working box or a refrigerated truck first if the temperature trend is upward. Food safety comes before diagnosis, and a technician can work on an empty box faster anyway.
Want it measured instead of guessed?
The readings that settle this — pressures, superheat and subcooling, amp draw, static pressure — take a visit. You get the price in writing before anything is replaced.
(904) 467-1100