SubZero Refrigerator Freezer Error Code 20 – How to fix? by KatyApplianceRepair.com 

Sub-Zero error code EC 20 indicates
a defrost underheat condition. Below is a quick reference
and troubleshooting guide for this code.

Quick Reference

Error Code	Meaning	Severity
EC 20	Defrost underheat — no voltage feedback through Gray/White wire at defrost start	Medium

Cause & Solution

Cause	What to Check	Fix
Defrost heater not producing enough heat	Inspect defrost heater wiring and connections	Repair wiring or replace the defrost heater
Faulty control board not sending defrost signal	Test voltage output from the control board during defrost cycle	Replace the control board if no signal is present