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Sub-Zero – Control Board Failure & Freezer Not Cooling

Professional Sub-Zero Refrigerator Repair in the Bay Area When a Sub-Zero BI-42UFD/S/PH suddenly stops cooling the freezer, most homeowners fear the worst — a failed inverter,…

Sub-Zero – Control Board Failure & Freezer Not Cooling

Professional Sub-Zero Refrigerator Repair in the Bay Area

When a Sub-Zero BI-42UFD/S/PH suddenly stops cooling the freezer, most homeowners fear the worst — a failed inverter, a burnt compressor, or a major sealed-system breakdown.
But this recent service call showed one of the most overlooked issues: no power going to the freezer compressor inverter due to a defective main control board.

This real-case repair from our Sub Zero Refrigerator Repair operations in the Bay Area is a perfect example of why proper diagnostics matter.

Symptoms

  • The freezer compartment was warming up, temperature climbing to 20–30°F, while the refrigerator side was still cooling normally.
  • Freezer compressor not starting
  • No humming, no vibration
  • Sub-Zero unit showed no error messages
  • All fans worked, lights worked, but freezer was “dead”

Initial diagnostics

Diagnostic Findings

During the inspection:

  • The inverter was tested — fully functional

  • Freezer compressor ohm’d out properly

  • Wiring harness clean, no burnt traces

  • Line voltage correct

  • BUT: the control board output for the freezer compressor showed 0V
    (should be 120V AC)

In other words, the compressor was healthy but the brain wasn’t giving it power.

This immediately points to a control board failure, a known issue on certain Sub-Zero BI models produced around 2020–2022.

Root cause of the problem

Why Control Boards Fail on Sub-Zero BI Series

These units often fail due to:

  • Power surges

  • Moisture intrusion near the upper panel

  • Internal relay burn-out

  • Aging components

  • Voltage fluctuations in older houses

When the control board stops sending 120V to the inverter, the compressor will not start and the freezer warms up silently — without error codes or alarms.

How we repaired the appliance

  1. Step 1 — Removed the upper service panel

  2. Step 2 — Removed the defective control board

  3. Step 3 — Installed the new updated OEM board

  4. Step 4 — Reset system, performed full calibration & verified output voltage to the inverter

    Confirmed compressor start-up and freezer cooling cycle, the inverter immediately received 120V, compressor ramped up, and the freezer began dropping temperature within minutes.

Models we service in this case

BI Series

BI-36UFD
BI-42UFD
BI-48UFD
BI-36S
BI-42S
BI-48S

PRO Series

PRO 36
PRO 48

FAQ – Sub-Zero Control Board Issues, Symptoms & Repair

Sub-Zero Refrigerator Repair

Sub-Zero freezer not cooling due to control board failure?

If your Sub-Zero freezer compressor is not starting due to
missing 120V at the inverter, the issue is often a failed control board.
Same-day diagnostics · OEM parts · Certified Sub-Zero technicians

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