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DSOS 0 Newbie Poster

Gateway 2.4GHz PC running WinXP SP2.

I noticed a few days ago that when I put my PC in standby mode the CPU fan will continue to run for as long as there is power to the machine. If I shut the machine down, of course it stops. A buddy suggested the temperature sensor may be failing/failed, but how do I know for sure, and if it is, what can I do about it?

I opened the case and vacuumed the dust out about 2 months ago, doing so I also removed the CPU fan and heat sink to clean all the gunk out. I didn't remove any of the heat transfer paste, but could it be that I need to apply more? Is it possible that my cleaning may have affected the cooling ability and it has slowly wrecked my processor?

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Dani AI

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A few focused checks for that save time and avoid replacing parts unnecessarily.

Most motherboards leave some power rails alive in certain ACPI sleep states (S1) so the CPU header can stay powered — that can make the fan keep spinning even though the OS is "in standby." Before blaming the sensor, try this short sequence:

  • Enter the BIOS/Setup and look for power-management or ACPI sleep options. If the BIOS supports S3 (Suspend-to-RAM) try enabling that; S1 often leaves fans powered while S3 usually turns them off.
  • Try Hibernate (Suspend-to-Disk) and a full Shutdown. If Hibernate stops the fan, the behavior is likely power-state related rather than a dead sensor.
  • Check BIOS health/monitor readings for CPU temperature and RPM. Then use a Windows hardware monitor (any well-known temp/RPM tool) to compare idle and load temps. A failed temp sensor usually shows nonsense (very low, very high, or no reading) or forces the fan to full speed all the time.
  • Reseat the heatsink if its mounting was disturbed: clean old thermal compound with isopropyl, apply a fresh small amount, and remount straight. Bad contact can cause high temps and aggressive fan behavior, but CPUs normally throttle or shut down before being permanently damaged.
  • Swap the fan to a different header or temporarily power it from a molex adapter to see whether the header/controller or the fan itself is the issue.
  • Disable Wake-on-LAN/USB wake and update BIOS/chipset drivers if ACPI behavior seems buggy.

Safety note: power down and unplug the PC (and ground yourself) before touching the heatsink or fan. If temperatures read normal but the fan still runs in sleep, it’s probably expected ACPI/bios behavior rather than a dying CPU sensor.

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