Most commercial controllers use NTC thermistor probes: small sealed sensors whose electrical resistance changes with temperature. A cabinet may have an air probe that controls the compressor, an evaporator probe that ends defrost, and sometimes a condenser probe that warns of overheating. Each sends its reading back to the controller along a thin cable.
Where the air probe sits matters. Placed in the return air, it reflects the average cabinet temperature; placed in the cold air stream from the evaporator, it reads colder than most of the stored food. Some cabinets for sensitive stock add a product-simulating probe in a small bottle of glycol or a solid block, which reacts slowly like real product and ignores brief door-open air spikes.
Probes fail occasionally, from water ingress, crushed cables or connector corrosion in humid conditions. A probe fault normally shows as an error code on the display. Replacing a probe with the exact type the controller expects is important, because a mismatched sensor gives wrong readings.