After disassembly and reassembly to fix an HDD cable issue, my iMac is now a blowhard - the fans are going at full tilt from about 30 seconds after boot. I don't believe the device is overheating - I surely messed up one of the temperature sensors upon reassembly.

My question - beyond carefully checking each JST connection and making sure I've re-attached all the sensor cables on the motherboard, is there any way I can control the fans via software? I believe I have more-or-less permanently damaged the skin sensor - so can I disable that through software configuration or a 3rd party utility?


It's not the HDD / SDD temp sensor, by the way, since it's not the HDD fan that's spinning that hot, but it certainly is one of the other sensors. The ODD fan's RPMs are strangely high, but it's the CPU fan that's out of control. Mac's Fan Control is definitely doing the job for now (I'm using the CPU heatsink temp as my RPM controller). I'll need to get in there and determine which of the sensors is the one acting up.

I wonder whether I can tell which sensor is acting up based on either A) the list of sensors appearing in the Mac's Fan Control sensor list (or, more precisely, the sensor that's NOT appearing in that list) or B) the temperature of a given sensor that's way out of whack.

  • 1
    Just clarifying: Did you replace the hard drive, or was it just the cable? Or something else? Also, it may be with editing your question to include the exact model of iMac you're referring to.
    – Monomeeth
    Commented Dec 6, 2017 at 23:38
  • 2
    Like this question and that question or this nifty solution?
    – bmike
    Commented Dec 6, 2017 at 23:47
  • I am just shooting in the dark, but if there is a thermal sensor in it, can you check that it is undamaged and properly connected? Also, can you check that the fan is connected in the proper connector? Commented Dec 7, 2017 at 6:58
  • I feel that my answer adequately responds to your question. Please consider accepting it as the answer by clicking on the checkmark next to it.
    – ProGrammer
    Commented Feb 5, 2018 at 8:52
  • I have a similar problem after swapping the motherboard of my 2010 27" iMac. CPU and ODD fans are running wild while the HDD fan is okay. I solved the issue with Mac Fan Control. But I have the feeling that the Mac is slower than it should be. I would be interested in the reasons why this happens. I had already swapped out the HDD and no such behaviour. It just now occured when I switched the motherboard and re-did the thermal paste on GPU and CPU.
    – bomben
    Commented Mar 8, 2018 at 7:52

2 Answers 2


The thermal sensor is indeed responsible for this and the issue (if we can even call it one) has been around for a long time. Two or Three years ago, I upgraded my iMac from the original HDD to a speedy SSD and had to find out the hard way.

It quickly became apparent that the thermal sensor causes this to happen and I had to find a way to reduce the screaming 6300ish RPMs of the internal fan down to the usual 2000-3000 RPM range.

After searching the internet for software, I came across a lightweight application called SSD Fan Control that comfortably resides in my Applications folder and doesn't require my attention except for cases where the iMac clearly requires a little more airflow. However, because it is located in one location (contrary to a laptop), I am able to estimate its airflow requirements very easily.

Other options such as Mac's Fan Control have more customisation options but for me, lightweight was key. Furthermore, it was important to have it set up once and not be bothered by it. SSD Fan control has never given me a popup, ads or asked me to upgrade to a new version. I, therefore, strongly recommend it and swear by its easy to use and efficient design and practicality.


Yes, there is a 3rd party app called Mac's Fan Control which allows you to adjust the speed of your fans. It gives you the current speed of your fans and has different options for setting the speed. You can set a variable speed which forces the fans to go faster as temperature goes up - as they are supposed to do.

there is also an app Temperature Gauge 4.1 that lets you monitor the temperature of different areas of your Mac and the fan speeds.

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