3 pin fan not being recognized on 4 pin cpu header

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leffreymays44

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Sep 23, 2014
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Hi I have just booted up my rig for the first time to get it to post and it would not due to the cpu fans I am running, corsair sp 120 3 pin fans , the fans are working but the system isn't recognizing them, I hit ignore in the boot menu for asus and have posted and installed windows, I did a reboot with the original cpu fan and heatsink the fan is a 4 pin and it booted without issues, do I need to replace my two 3 pin cpu fans for 4 pin fans?

thanks
 
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what Asus board are you actually using?
 
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For any modern MB with 4-pin PWM CPU fan headers, it's always going to be best/easiest to run 4-pin PWM fans for your CPU cooler.

With an ASUS MB, you can turn off the fan alert (IGNORE) or set a lower speed which may solve your POST problem. You would also need to find the setting that allows 3-pin fans to be controlled with variable voltage from a 4-pin header instead of the PWM control the header is designed for. How to do that (and even if you can do it) will be specific to a particular motherboard.
 
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You can try to update the BIOS. I had problems with Asus motherboard with z97 chpset (I dont remember the model) that has problem with the stock Intel fan. After BIOS update the problem was fixed.
There is an option in newer BIOSes that defines the minimum RPM of the fan. If it is too high it can cause problems. Try to reduce it to the minimum.
 
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Page 3-27 of your manual:

Plug the CPU fans into the CPU header, not the CPU OPT header.

Set Q-FAN CONTROL to ENABLE

SET CPU FAN PROFILE to STANDARD

SET CPU FAN SPEED LOWER LIMIT to 200 RPM

This last one is probably your problem. These MBs default to stock fan cooler setting with small fans that run at very high speeds. They quite likely start up faster than 600 rpm. Same signal to your large slow fan might be below 600 rpm, triggering the alert. Setting it to 200 rpm should be low enough while still giving you protection.

If that doesn't work, for testing purposes, you could set it to IGNORE, which skips the whole fan speed test. I wouldn't like to leave that as a permanent setting because you would be unprotected if, for example, you forgot to plug in the fans some day. But, for testing, it might help sort things out.
 
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I never saw an option for 3 pin fans in the bios and did try and lower the speed fan and still wouldn't post I finally hit ignore because I know they are cooling but they will not have speed control.
 
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Just a little background. 4-pin PWM fans will usually start at 20% of max power. 3-pin voltage controlled fans are only going to reliably start at 30% to 40% of max power (4 to 5 volts)

I believe that the stock setting for the ASUS boards is a setting of 20% (it assumes a PWM fan). Even if your fans start, 20% speed of your fans is just 470 rpm. So if you have your MB set for a 600 rpm fan alert, you are going to get an alert because your fans are turning too slowly. That's why they give you the option to lower the alert speed. 200 rpm should be OK while still protecting you with a POST-failure alert if the fans aren't running at all.
 
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I think your MB will be able to speed control either 4-pin or 3-pin fans. It can certainly speed control 3-pin fans connected to the CHASSIS fan headers. The STANDARD fan profile should give you variable fan speeds based on CPU temp, but we need to solve your POST problem before getting into all that.

Did the IGNORE setting allow the computer to POST with your 3-pin fans?
 
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Yes With ignore selected it boots perfectly, funny thing though after I did that boot up with the stock 4 pin fan now it will boot just fine on the 3 pin cpu fans but The bios still shows that it isn't seeing a cpu fan I am not sure right n ow if I have ignore selected. I will have to check it out later when I get out of work.
 
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If you are using a splitter from the CPU fan header to your two Corsair fans, make sure the splitter is properly built with a speed sensor pin on only ONE of the two fan connections. The third pin should be missing on the splitter connector for one of the fans. The CPU fan header can only deal with a tach speed signal from ONE fan. It will be all confused if it's getting a TACH SPEED signal from two fans at the same time.

Most splitters take care of that automatically, but there are some crappy ones that have the third pin for both fans. You have to break the pin off for one side of the splitter to fix the problem. It's usually (but not always!) the yellow wire from the fan. Red (+), black (-), yellow (tach/speed).
 
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I did some reading on fan splitters and read just what your telling me, do you think a four pin splitter would correct this cpu fan issue, right now the two fans are hooked up to cpu fan and cpu opt headers.
 
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It shoudn't make a difference, but it might. There's a note in your MB manual that says Fan Xpert features are not available for fans connected to the CPU_OPT fan header. Fan XPERT is the fan control software than can be downloaded for your MB. I'm not sure why that NOTE is there or what it means. I don't see anything in the pin outs that suggests that the two CPU fan headers should be different.
 
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Maybe, but my guess is that he CPU_FAN and CPU_OPT headers are simply two headers split off the same controller (like a splitter built into the MB). I think what they mean by the Fan XPERT note is that, when you control the CPU_FAN header, you are controlling both of the CPU headers at the same time. That's just a guess.

You could easily test it by booting with just one of the fans connected to the CPU_FAN header. Plug the other one into a CHA_FAN header if you want to so it runs while you are doing a clean test with just one fan plugged into the CPU_FAN header.
 
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Just use it without a fan plays games very well just forget about the heat and temps!
i am using a intel i7 5950x with no cooler and runs at 80 c idle.And in games 120c!
Its safe man try it!
 
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