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After upgrading to High Sierra (I have clean installed it), my mac (Macbook Pro 2010) needs unreasonably longer time to login. As a result, I have noticed that the temperature increases to around 100 C during the startup time (I have checked this right after login through Istat Menus). Meanwhile the fans are at 2000 rps. Apparently , the reason why fans do not speed up is because the system as a whole is within the safe temp with the only exception of the CPU temperature as CPU is busy processing all those startup processes. So the fans assume everything is alright and do not speed up.

Now my question is if this causes any harm to CPU as I think I read somewhere that quick fluctuations within a short range in the temp of CPU can cause damage to the CPU. I just wanted to know your ideas on this.

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In iStat Menus, check the figure for 'relative to ProcHot' - so long as this is a positive value, all is OK in theory, though I would have expected the fans to ramp before that temperature was reached.

Maybe try resetting the NVRAM & SMC - Really confused about how to reset SMC and PRAM and NVRAM on my Macs and iPad and iPhone? has a comprehensive answer on how to do this for all Mac models.

Picture of where is shows on my machine, but as it's a dual processor Mac Pro no doubt it will look very different. [bottom half menu cropped off, to save space]

enter image description here

  • Thanks for replying. But I can't seem to find the figure for 'relative to ProcHot'. – developer Oct 28 '17 at 9:58
  • It might be different per machine, not certain. I'll post a pic of mine - but mine is a dual processor Mac Pro so no doubt it will look very different to yours... – Tetsujin Oct 28 '17 at 9:59
  • Thanks for the picture, but no I do not have such a figure. By the way my P-bus is always around 12.15 V. Is that normal? – developer Oct 28 '17 at 10:07
  • Sorry, i guess we'll have to wait for someone with specific knowledge of MacBooks - I've never owned one. – Tetsujin Oct 28 '17 at 10:41
  • Did you reset the NVRAM & SMC anyway? – Tetsujin Oct 28 '17 at 10:47
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Now my question is if [the CPU being hot versus the average temp of the reset of the MacBook] causes any harm to CPU as I think I read somewhere that quick fluctuations within a short range in the temp of CPU can cause damage to the CPU.

No.

Your MacBook will do multiple things to address the CPU temp from throttling (kernel_task process) the CPU to spinning up the fans (provided you don't have another program overriding the fans' operation).

This has already been addressed on this site: kernel_task hundreds of % CPU yet cpu frequency is being lowered

If your CPU goes past its temperature threshold, it will shut itself down (outside the influence of the OS) in a process aptly named "thermal shutdown."

Apparently , the reason why fans do not speed up is because the system as a whole is within the safe temp with the only exception of the CPU temperature as CPU is busy processing all those startup processes. So the fans assume everything is alright and do not speed up.

The fans assume nothing. They also don't operate on the basis of an mean system temperature. If it (SMC) sees the CPU is too hot and/or under load, the SMC will spin them up unless something (like SMCFanControl) is overriding the default behavior.

If you've ever looked inside a MacBook Pro, the fan which is attached the heatsink covers only two components - the CPU and, the GPU. The SMC isn't going to try to cool the CPU with a fan directly attached to it by waiting until the other temperatures spike.

enter image description here

Instead of looking at iStat, it would be more beneficial to look at Activity Monitor to see what the CPU is doing. Try running in Safe Mode (hold Shift while booting) Recovery Mode, or booting from an external disk with El Capitan or Sierra installed. If the problem goes away, the issue is with High Sierra, not your MacBook.

  • Thanks for replying. But I have noticed that I do not have the same problem (similar to the high cpu temp at startup with low fan speed) when the body of the mac gets moderately hot. I mean, with the latter condition, when the cpu temp goes up, so does the fan speed and the fans run properly and in accordance with the cpu temperature or any that of any other component when they reach the threshold. The problem based on my experience seems to be somehow related to the mac's mean temp. – developer Oct 28 '17 at 13:20

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