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Bug: CPU fan loses control after wake from S3 sleep or S4 hibernate

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Domain
devtools

Research direction

Start by reproducing the reported loss of CPU fan control after waking from S3 sleep or S4 hibernate on Windows 11 with the listed Gigabyte motherboard. The report says restarting FanControl.exe does not restore control, while a full Windows restart does; investigate how the built-in LibreHardwareMonitor layer handles hardware access across wake. Done means normal control resumes after wake without a full restart.

Written by the indexing model from the issue text.

Description

Bug: CPU fan loses control after wake from S3 sleep or S4 hibernate

Fan Control version: V266
OS: Windows 11
Motherboard: Gigabyte X870I AORUS PRO ICE (AM5)
Sleep states available: S3 (Sleep) and S4 (Hibernate)
Plugins installed: None (built-in LibreHardwareMonitor only)

Description

After waking from either S3 sleep or S4 hibernate, Fan Control loses control of the CPU fan header. The fan gets stuck at ~1500–1580 RPM instead of the expected ~800–900 RPM at 35% duty cycle. Fan Control shows a divergence warning stating the control does not respond correctly to the fan curve. The issue persists until a full Windows restart.

Expected behavior

At 35%, the CPU fan should spin at ~800–900 RPM per calibration data. Fan Control should resume normal control after wake.

Actual behavior

After wake, the CPU fan is stuck at ~1580 RPM (~50–60% equivalent), regardless of what Fan Control commands. The hardware EC appears to take over the fan header and ignore Fan Control entirely.

Tests performed (all failed)

  1. Enabled "Forced Control" option in the Fan Control divergence dialog
    → No effect. Fan stayed at ~1580 RPM.

  2. Set CPU fan header to Manual mode in BIOS (Smart Fan 6 settings)
    → No effect. Same divergence warning after wake.

  3. Manually killed and restarted FanControl.exe after wake (via PowerShell Stop-Process + Start-Process)
    → No effect. Fan remains stuck even after a fresh process start.

  4. Switched from S3 sleep to S4 hibernate (full power-off state)
    → No effect. Same issue occurs on wake from hibernate.

  5. Checked for kernel driver services related to Fan Control (WinRing0, LHM, etc.)
    → None found. No separate driver service running.

Key observation

A full Windows restart is the only action that restores normal fan control. Neither restarting the Fan Control process nor switching sleep modes resolves the issue. This suggests the hardware EC on the Gigabyte X870I locks the fan header on wake in a way that Fan Control's LibreHardwareMonitor layer cannot recover from without a full driver reload (which only happens on a clean boot).

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