Excessive heat from MacBook Pro under MacOS ‘Mojave’

My MacBook Pro (a late-2013, 39 cm model, MacBookPro11,2) have been running hot after I installed the first developer preview release of MacOS 10.14 “Mojave”. The software, and fans and ventilation seemed fine; but what about the power management firmware?

I initially ignored the problem and attributed it to either the higher ambient temperature caused by the heatwave currently sweeping across Northern Europe. Or maybe it was a pre-release bug? However, as the fourth beta release came and went I started to look into the issue in more detail.

The processor temperature, measured with the Intel Power Gadget, when the computer was under no load and idling (processor utilization under 5 %) was stuck at around 70℃. The high base temperature makes the machine hot to the touch, and also causes thermal throttling of the processor when you want to get something done.

I thought that the internals fans or air passages might be blocked by dust and debris. The machine would get hot mere minutes after boot, and it sounded like the fans operated normally. However, this model MacBook Pro has received a meager iFixit repairability score of 1 out of 10 and I didn’t even want to attempt to take it apart if I could avoid it.

I wasted quite some time trying to catch an app using a lot of resources in bursts with the Energy usage view in Activity Monitor; beveling there was a specific program causing the my issues. As it turned out, there were no rouge crypto-miners or apps experiencing unexpected resource demands with the MacOS beta.

I then stumbled upon an Apple Support document that suggested resetting the System Management Controller (SMC), which is responsible for “thermal management”. This is an easy enough process and Apple Support provides detailed instructions for resetting the SMC on a variety of Mac models.

After resetting the SMC, the CPU temperature dropped from 70℃ when idling to just shy of 40℃, and normalized around 45–65℃ with normal use. I haven’t had any issues with my MacBook running excessively hot after resetting the SMC.

There’s little good documentation available on what the System Management Controller does and how it works. The SMC is responsible for power distribution and thermal fan controls, so I can only conclude that it was giving either the processor or some other controller too much juice that just turned into waste heat.

I didn’t think to measure the battery life impact, but it stands to reason that also should have improved given the reduced cooling needs.

I find it interesting that such a simple fix would have much of an impact on the thermals of a modern computer. If it’s a known problem that the SMC gets intro trouble, and Apple have even documented how to reset it: shouldn’t it just reset itself periodically on reboot? I see no reason why it shouldn’t just reset itself automatically every 20 days or so to save people from whatever goes wrong with it. How many MacBook Pros out are running unnecessarily hot or slow over some silly maintenance task that no one knows they’re expected to perform every now and then?