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Why Is My Computer’s Fan Coming On?

Question: My computer fan kicks in for no reason. It never did this before, but it does now. I know that a dirty computer fan could cause overheating, but I checked and blew out the fan with compressed air. Nothing came out. What could be the problem?

The problem is still heat-related. The question then becomes: where’s that heat coming from?

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The CPU and fan

It sounds like you know this already, but let’s review. As you work on your computer, your CPU generates heat. There’s a fan inside your machine that keeps it cool. When the fan is clear and the inside of your machine is clean, then the cool air has a chance to flow through and keep the CPU temperature under control.

The most common problem with a computer fan is that dust accumulates inside your machine and prevents the fan from blowing cool air through the computer. If that’s blocked, then the heat that’s generated by the CPU just accumulates and never cools down enough for the fan to turn off.

Another reason why your fan might be coming on could be that the CPU is being heavily used. Like I said, the CPU generates heat as you work. It generates more heat the harder it works.

When the heat is on

So how or why could the CPU be generating heat now? For some reason, it’s working harder than it has in the past.

That could be normal use. If you’re running something that’s very CPU intensive, like video decoding or encoding software, for a long time, I would expect the fan to kick in because the CPU generates heat from all of that work. It might be unexpected, but it’s still normal use.

On my MacBook Pro, I often run Parallels so I can run a Windows virtual machine. As it turns out, that’s just enough extra processing that even though I’m doing nothing, the fan on the MacBook will kick in every once in a while.

It surprised me. If I turned off Parallels, then the fan never kicked in. It’s normal in the sense that it’s the result of something that I was running intentionally, but it was unexpected.

Computer Power Supply and FanThe CPU, the fan, and malware

The other reason why the fan is running could be related to malware.

Depending on what it wants to do, malware can cause your computer to start working in ways that make the CPU work harder.

The solution is to get rid of the malware. If this is the case, make sure:

  • Your anti-malware software is running up to date.
  • Your firewall is in place.
  • You’re doing all of the right things to keep your computer safe on the internet.

Of these scenarios, I don’t really know which one is happening to you. Check out my article, “Who’s hogging my CPU?” which will actually show you how to use a tool called Processor Explorer (that you can download for free from Microsoft and run). It will show you which processes and programs on your computer are actually using the CPU.

If you find that the CPU is spending most of its time idle, then it’s not the CPU heating up. It’s something else. You may find that there’s a process or a program that’s using more CPU than you expected and from there, you can make some decisions.

Ambient temperature

I will throw out one last comment. Ambient temperatures, or the temperature in the room, can contribute to this.

Normally, we’re talking about the temperature in a room being in the upper 60s Fahrenheit with CPU temperatures well into the 100s. If the room is 10 or 20 degrees warmer because you’re in the middle of the heat wave, then it’s possible that the entire computer could be just warm enough to push it beyond that threshold where it says, “You know, I should really turn on the fan.”

Those are the ideas that come to mind. Still, you took care of one dust issue. Next is to look at what’s using your CPU now.

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16 comments on “Why Is My Computer’s Fan Coming On?”

  1. I find that my fan will often run at high speed if there is an update that has been downloaded that requires a restart of my computer, but the restart hasn’t happened yet. The solution is (obviously) to restart the computer. I’m not sure what the technical reason for this is – it could possibly be causing high use of the CPU.

  2. Another thing that causes overheating is when the “grease”, or thermal paste, between the CPU and heat sink dries out. This usually happens when the manufacturer applies too much. For more info do a search for Arctic Silver 5.

  3. A few years ago at the office we were having an epidemic of laptops (all the same brand and all purchased at the same time) that were having overheating problems. The culprit turned out to be the material that connected the CPU to the heat sink. The material was defective. It had shrunk and cracked and was not providing proper transfer of heat from the CPU to the heat sink. Our tech put in new material and the overheating problems went away.

  4. Whilst agreeing with all, additionally make sure that the various ventilation grills or louvres are not blocked, either by dust similar to the fan blades; or being blocked by other objects such as books, magazines, paper and the like BUT ALSO WALLS etc by being placed too close to them.

    I have attached a 1 inch square section of wood by Velcro Hook & Loop tape to keep my tower PC clear of walls.

    My two laptops are always used on a hard surface such as a table or a tray, to prevent similar blocking to the ventilation slots on their under-sides.

    Additionally when checking the CPU Heat sink, make site that its heat-dissipating fins are clean.

    • When I use my laptop on my lap (as infrequently as possible) I have a wire stand that helps give it some space and ventilation from my lap. Otherwise it gets way too hot.

  5. It is not clear from the original question if the person has a desktop or a laptop computer. If a desktop then “blowing out the fan with compressed air” is definitely not enough to ensure that the computer is dust free. The case should be opened, and the cooling fins of the heat sink need to be blown out directly, along with any other accumulated dust & lint inside the case. If you have never opened up a desktop case, you would be amazed at the amount of dust, dirt, cat hair, dog hair, etc. that can accumulate inside!

    I would agree with others who have indicated that the thermal paste layer between the CPU and the heat sink should also be checked and replaced if it is dried out and no longer effective.

  6. I don’t know if you have a separate GPU, but if you do, that’s a good place to check. I had a fan on a video card conk out, the GPU heated up to over 60 degrees C, and then it predictably conked out–but not before the ‘ambient temperature’ rose significantly!

  7. All good information BUT… Cleaning I know about… I need to know how to keep my 17 year old cat from sitting on the nice warm CPU where the exhaust openings are . I am afraid of over heating.

    • If it only happens with ROBLOX. it sounds like maybe that ROBLOX is more graphic intensive and requires more resources than other programs.

  8. So…I have a laptop. I accidentally splashed water on the key board. Hours later, I go to turn it on, but only the fan turns on. Is the fan on, to dry the moisture inside the computer?


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