Modern refrigeration is run by controllers, and a controller reading a failed sensor will do exactly the wrong thing with complete confidence. JRC diagnoses control faults across Elma, IA.
A box that will not start, one that runs continuously, or one that defrosts at odd hours often has nothing mechanically wrong with it. The temperature probe has drifted, a pressure switch has failed open, or somebody changed a setpoint and nobody wrote it down. We read the actual sensor values against a calibrated thermometer rather than trusting the display. Where controls have been replaced piecemeal over the years we will document the setup so the next technician is not starting from nothing.