How does the CPU know it's temperature? I've been having trouble turning on my computer as I keep getting "CPU fan error". I reapplied thermal paste and got a new heatsink/fan and still the same problem. Could it be a false positive? How can I tell? Any other causes of the error that don't actually have to do with the CPU heat, for example could the CPU not be correctly seated? The heatsink I'm using is stock Intel and I heard all 4 pins click in. Plus when I took it off the thermal paste had been smudged around so I knew it made contact, and I watched the fan spin.

UPDATE: it's working now. I didn't know it made a difference where you plugged in the power cable to the motherboard. I had tried plugging the fan into two connectors, and though the fan turned on, I guess the same channel is used to convey information about CPU temperature? Here is a picture with red arrows pointing to the power connectors (or what's the correct name?) where I plugged in the fan and had it turned on but got the CPU fan error. Interestingly there is a fan on the side of the case, which I unplugged and tried to plug the heatsink fan into, and it still didn't work. I guess there's something very special about the connector it's plugged into now. Thanks for the help everyone!

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    Sounds more like the motherboard can't detect whether or not the CPU fan is spinning. Is your CPU fan connected to the motherboard connector specifically designated for the CPU fan? Can you get into the BIOS menu and is it detecting the fan RPM there? – BrianC May 4 '16 at 0:34
  • The CPU fan error is generally caused by low voltage to the fan or damage to the blades. – Burgi May 4 '16 at 0:57
  • Just to rule out user error, is it actually plugged into the right header? – Journeyman Geek May 4 '16 at 1:01
  • Its unlikely the CPU fan error relates to the CPU temperature - the former would be sensing voltage or equivalent on the motherboard, the later is a heat sensor built into the CPU itself. – davidgo May 4 '16 at 1:43
  • "How does the CPU know it's temperature?" Modern cpu's have a built in temp sensor. – Moab May 4 '16 at 1:53

Basically there's multiple headers for fans on a motherboard, and while electrically and functionally identical, the CPU header is monitored seperately, since a non functioning CPU fan would be very bad. If you plugged it into the "aux" or "opt" header - meant for things like "push pull" cooler configurations or water pumps or a "cha" header, meant for chassis fans, the motherboard might decide "hey, the CPU fan isn't taking any power or reporting speeds, it must be dead" and warn you that the CPU.

As such when building a system where you plug in a fan matters, especially for automatically controlled fan speeds, and to prevent error messages like this.

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