Sub-Zero Code 20: Defrost did not heat correctly

What 20 means

On legacy Sub-Zero 600 Series models like the 650/F, 685 and 690, plus 700 Series, 736 and IC-27 units such as the 736TCI, code 20 reports a defrost underheat: at the start of defrost the control saw no voltage returning through the gray/white wire, which points to the freezer defrost terminator. On the legacy 648PRO the same number means the defrost bi-metal in zone 1, 3 or 4 is not operating properly. In both cases the coil may not clear its frost, and cooling slowly declines.

Common causes of 20

Defrost terminator or bi-metal not closing
Break in the gray/white feedback wire
Loose connection in the defrost circuit

✓ One safe check you can try

1) Note the code and, on a 648PRO, the zone number. 2) Look at the back wall of the freezer for heavy frost. 3) Keep doors closed to slow frost buildup. 4) Turn the unit off and on at the breaker once. 5) Arrange service if the code stays.

! Why to stop there

The defrost terminator sits on the evaporator behind interior panels.
Checking the feedback circuit requires voltage tests.
A frozen coil needs controlled thawing.

Opening a panel yourself does not automatically cancel a manufacturer warranty, but it can cause accidental damage, and some manufacturers set their own service conditions. Check your warranty terms, or ask us, before deciding.

→Same appliance

Other Refrigerator codes

41Sub-Zero 648PRO Code 41: Zone compressor or wiring problem 81Sub-Zero 700-2 Code 81: Variable-Speed compressor fails to start 82Sub-Zero 700-2 Code 82: VS compressor controller needs replacement 88Sub-Zero 700TF Code 88: Board and compressor controller not talking 80Sub-Zero 700TF Code 80: No inverter feedback after a board change ddSub-Zero 600 and 700 Series Display dd: Diagnostics switched off deSub-Zero 600 and 700 Series Display de: Diagnostics enabled 23Sub-Zero 600, 700 and 648PRO Code 23: Defrost overheat with no feedback

2020 questions

Does code 20 mean the same on a 600 Series and a 648PRO?

Close but not identical. On 600, 700, 736 and IC-27 units it is a defrost underheat with no voltage feedback from the terminator. On the 648PRO it is the defrost bi-metal in a specific zone not operating.

Why is my freezer frosting up?

If the defrost circuit does not complete, frost stays on the coil and blocks airflow. Over time the freezer warms even though the compressor keeps running.

What do you replace?

Usually the terminator or bi-metal, or we repair the gray/white feedback wire. We confirm with tests first and quote the repair in writing before starting.