SubZero Refrigerator Freezer Error Code 24 – How to fix? by KatyApplianceRepair.com 

Sub-Zero error code EC 24 indicates
a defrost underheat condition. Below is a quick reference
and troubleshooting guide for this code.

Quick Reference

Error Code	Meaning	Severity
EC 24	Defrost underheat — heater not reaching required temperature	Medium

Cause & Solution

Cause	What to Check	Fix
Defrost heater weak or failed	Measure heater resistance; inspect wiring to control board	Replace the defrost heater if resistance is out of spec
Defrost terminator opening prematurely	Test terminator open/close temperatures	Replace the defrost terminator
Control board not sending full defrost cycle	Monitor defrost cycle timing with a multimeter	Replace the control board if cycle is shortened