Sub-Zero built-in refrigerator (Classic/BI and earlier 600 series): field service manual

Two independent sealed systems: each section has its own compressor, evaporator and evaporator fan, controlled by one main control board that reads a cabinet thermistor and an evaporator thermistor per section. Because the systems are separate, a 'refrigerator warm, freezer fine' complaint points at that section's own fan, defrost, thermistor or compressor relay, not at a shared damper as on GE Monogram or most mass-market units.

Condenser and airflow: the condenser and condenser fan sit behind the kick/top grille. Sub-Zero asks for cleaning every 6-12 months; a loaded condenser makes the compressors run long, which the control flags as 'Vacuum Condenser' (or 'Service') on models that have that message.

Defrost: each evaporator has a defrost heater with a bi-metal safety thermostat wired in series. The control ends defrost on evaporator thermistor temperature; if the evaporator does not warm as expected, it logs a defrost 'under-heat' code (family 20, for example 20 2 07 on Classic/BI).

Fault logging: the control stores codes in memory. On 600/700 series a steady (not flashing) 'Service' message means codes were seen in diagnostic mode and not cleared; a flashing wrench with chiming on Classic/BI means a major code is logged. Some codes are hidden from the display after software updates but stay readable in service mode and in Sub-Zero's Connected Diagnostic Program (CDP), the dealer/authorized-service software tool.

Special modes: Showroom mode (lights and display on, no cooling) and Sabbath mode (no temperature shown, panel not responding normally) both exist and both mimic a failure. Check them before anything else on a 'not cooling' or 'dead panel' call. Entry steps vary by model: use the owner's guide for that model.