Sub-Zero Error Code 10 F 00: Freezer Evaporator Thermistor Open (BI Series)

On a Sub-Zero Built-In (BI) series unit, error code 10 F 00 means the freezer evaporator thermistor is reading as an open circuit. This sensor monitors the temperature of the evaporator coil in the freezer, which the control uses to manage cooling and, importantly, the defrost cycle. An open reading means that input is lost.

This is a sensor-circuit fault rather than a refrigerant problem, but because the evaporator thermistor influences defrost timing, an unresolved fault can lead to frost buildup and poor cooling. The recommended diagnosis is to check the thermistor's resistance (ohms) and verify it against actual temperature, then inspect the wiring and connector. This is technician-level work.

The meaning of 10 F 00

The evaporator thermistor changes resistance with coil temperature so the control knows when the evaporator is cold (cooling) and when it has warmed enough to end a defrost cycle. An open reading means the control sees a broken circuit from this sensor (effectively infinite resistance). Causes are typically a failed thermistor, a disconnected or corroded connector, or a broken harness wire. Because this sensor is tied to defrost control, an open circuit can cause frost accumulation on the evaporator and reduced freezer performance.

Most-likely causes

• Failed freezer evaporator thermistor (internally open)
• Loose, corroded, or disconnected connector at the evaporator sensor
• Broken or chafed wire in the harness, sometimes from ice or frost stress
• Frost or moisture intrusion causing corrosion at the connection
• Control board input fault on the thermistor circuit (less common)

Safe owner checks

• Power cycle the unit at the panel or breaker; a true open will return but a transient glitch may clear.
• Watch for heavy frost buildup or reduced freezer cooling and note it for the technician, since this sensor affects defrost.
• Do not open the evaporator panel or disturb wiring yourself; the evaporator area involves frost and delicate connectors and should be serviced by a technician.

When professional service is needed

Confirming an open evaporator thermistor requires measuring resistance against a temperature/ohms chart and inspecting the harness and connector behind the freezer evaporator panel. Because this sensor governs defrost, correct diagnosis and replacement, and clearing any resulting frost, should be done by a qualified technician.