WALK-IN CHILLER REFRIGERATION & FREEZER UNITS: check that coils and fins are in good shape, check fan blades and compressor unit motors, check thermostat settings vs. actual temperatures inside unit, check for leaks of any kind, check oil and refrigerant levels, clean and remove all sand and dust (inside and out), check defrost timers for appropriate activation and duration times, check breathers and wiring, check all pressure switches, check all voltage readings