Sub-Zero Code 24: Defrost under-heat

What Code 24 Means

Defrost System Fault

Defrost under-heat

Defrost codes mean the heater or thermostat responsible for melting frost off the evaporator coil during the defrost cycle isn’t behaving the way the control board expects.

01
The defrost heater has failed open or is drawing too much current (shorted)
02
The bi-metal defrost thermostat is stuck open or stuck closed
03
A wiring fault between the control board and the defrost circuit

Safe Checks You Can Do First

Before you call.

A few things worth checking yourself — none of it requires opening the sealed system or working inside the control panel.

01
Check for visible frost buildup if you can safely see the evaporator area
02
Note whether the unit seems to run through a defrost cycle at all — brief warming or dripping sounds — or never seems to

When to Call a Professional

Confirming a defrost fault means checking the heater’s resistance and the bi-metal thermostat’s switching temperature with a meter, both of which sit behind panels that require at least partial disassembly.

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FAQ

Code 24, answered.

What does Sub-Zero code 24 mean on my refrigerator?

Code 24 means: Defrost under-heat. It falls under defrost system fault — Defrost codes mean the heater or thermostat responsible for melting frost off the evaporator coil during the defrost cycle isn’t behaving the way the control board expects.

Can a defrost fault clear itself?

No — a failed heater or stuck thermostat is a hardware issue that needs to be repaired, not something that resolves on its own.

Is it safe to keep using the unit with a defrost fault active?

Short-term yes, but frost buildup will worsen over time and eventually restrict airflow enough to affect cooling, so don’t leave it too long.

Can I manually defrost the unit myself while waiting for a repair?

You can turn the unit off and let it defrost naturally as a temporary measure, but that doesn’t fix the underlying heater or thermostat issue.

Does a defrost fault always mean a new heater?

Not always — it could be the heater, the bi-metal thermostat, or the wiring between them, which is exactly what the diagnostic visit determines.

How long does a defrost heater repair typically take?

Once diagnosed, it’s usually a same-visit repair, though access can add time depending on the model.

Sub-Zero Code 24: Defrost under-heat

The fix points to the defrost heater or bi-metal thermostat. Confirming a defrost fault means checking the heater’s resistance and the bi-metal thermostat’s switching temperature with a meter, both of which sit behind panels that require at least partial disassembly.

Defrost System Fault

What you might notice.

Before you call.

Confirming a defrost fault means checking the heater’s resistance and the bi-metal thermostat’s switching temperature with a meter, both of which sit behind panels that require at least partial disassembly.

Other Defrost System Fault codes.

Code 24, answered.

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