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I was plugging in my computer and I noticed that one of my case fans wasn't spinning and the other one was. I opened up the case and spun the fan with my hand and it started spinning again.

I thought it might be something to do with the connector on the motherboard so I disconnected both case fans and connected the fan that was working properly to the connector that the broken fan was connected (and vice-versa for the broken fan). The broken fan still didn't spin (while the other fan did) so I guess its safe to rule out a problem with the motherboard.

I'm wondering if I need to replace the case fan and if so, do I buy a whole new case fan or just the fan? The case fan uses a 3 pin connector, but I noticed another cord on the case fan that goes to the back of the case. Any idea what that's for?

The case fans came with the case and the case is the Antec Three Hundred Two. Not sure if its needed, but the motherboard is MSI Z97-GD65.

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    Replace the fan with a fan with the same size but higher CFM (cubic feet per minute). a 12cm fan with +100 CFM is a proper fan for case.
    – user326006
    Mar 28, 2015 at 7:50

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3 pin fans are voltage controlled only, feed less voltage to them to get them to slow down. An older fan that the lubrication had not been maintained, or just the usual cheap Sleeve bearing fan would have trouble starting up at lower voltages, although it might start-up properly at full voltages. Once started (as you noticed) it will run.

Antec has/sells fans that have a speed adjuster external to the fan, if you see wiring going to the back of the case, it could be a switch exists there, on the case to adjust speed? if it has a speed adjuster, setting it on high(er) will have it start up in lower voltages.

Assuming your connecting these fans to the motherboard , the motherboards are capable of thermally controlling the fans speed through Voltage changes. Low RPM antec case fan, plus if it is set on low, plus the thermal control of the motherboard and it does not start. Still given enough voltage it would start, and they do not really burn out from being too low and trying over and over again and not spinning.

You could replace the fan with a better ball bearing style, or adjust the motherboards software control bios/uefi to a higher setting, or switch the fan to a higher setting (if that exists).
The only real problem that would exist would be if there is not enough cooling when thing get heated up.

If instead the fan(s) are connected to an antec speed controller on the case, or a thermal speed controller, then you could adjust the cases own controller, because the motherboard is (probably) not a part of the scenario.

Conclusion (so far) it is voltage controlled, it is low(er) rpm fans and being fed too low of a voltage to start-up. Ignore it if it still works when needed. OR Tweak the controller to a higher speeds so it will start-up. OR just use a different fan.

Other important notes: Most of the controllers will start up with full voltage, so usually when Posting (booting) the fans would roar up to full speed once, which does get them moving, then the thermal control kicks in lowering the speed. If you stop or disconnect a fan, you would have to "Post" again before you would know how it would act in real situation.

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    I looked at the back of my case and saw that they are fan speed controls and they were both set to low. I changed them to high and the fan that wasn't spinning, spins now. Also, before the other fan would only spin once Windows was booting. Now they both spin right when I turn on the computer. Thanks! Mar 30, 2015 at 1:51

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