Power Your MacBook Pro From The Right Side USB-C Ports: Here’s Why

Right-side USB-C ports on a 16-inch MacBook Pro. Photo via hardware zone.com.sg
Right-side USB-C ports on a 16-inch MacBook Pro. Photo via hardware zone.com.sg

Do you have a recent MacBook Pro? While these powerful notebooks have USB-C / Thunderbolt 3 ports on both sides, we’re going to tell you why you should charge your MacBook Pro using the ports on the right side.

On StackExchange and other sites, MacBook Pro users documented situations where charging using the left side USB-C ports caused unusually high CPU usage. The result? The machine would slow down while the fans on the device went into high gear.

The cause of this sudden quirk? Adam, a user on StackExchange, came up with a way to test what was happening and fully documented his results.

He discovered that if you’re charging your MBP using the left-hand ports and have other accessories plugged into that side, the computer heats up so much that a sensor — Thunderbolt Left Proximity — is alerted and a macOS process named kernel_task appears. Suddenly the fans spin up on the MacBook Pro and CPU usage is maxed out.

Image via "Adam" on StackExchange showing the macOS kernel_task dominating CPU usage
Image via “Adam” on StackExchange showing the macOS kernel_task dominating CPU usage

Adam’s graphed test results are too big to post here, but they conclusively show that plugging in power on the right side resolves this runaway state. He describes the test as follows:

State A a USB-C hub (a mouse and keyboard, plus power) and a USB-C HDMI 2.0 adapter, both on the left side. You can see the Thunderbolt Left Proximity temperature sensor rise quickly. About 3-4 minutes later the dreaded kernel_task high CPU usage starts.

State B cures the kernel_task problem by moving power from the left ports to the right. The left side temperature drops and the kernel_task goes away within about 15 seconds.

This is causal. Moving power back to the left side, restoring State A, quickly restores the temperatures and kernel_task again comes back after 3-4 minutes. Again moving power back to the right side, restoring State B, resolves the problem immediately.

Ergo, high CPU usage by kernel_task is caused by high Thunderbolt Left Proximity temperature, which is caused by charging and having normal peripherals plugged in at the same time.

Adam on StackExchange

Oddly enough, I personally ran into this problem a few months ago while reviewing a USB-C webcam. Not knowing about this issue, I believed it was the apps that came with the webcam that were causing the fans to spin up and the machine to slow down. A quick test today showed that it was resolved by powering the machine from one of the right USB-C ports.

The things you don’t know can hurt you!



LEAVE A COMMENT


  • So when we refer to charging a MacBook Pro 16″ from “the right side” [so that the fans do not go into high gear], looking directly at an open laptop, which is the right side which is referred to (which we should use)? Does “the right side” correlate to our right side/right hand when facing an open laptop (which is really the left side of the laptop when closed & facing away from us?
    So the computer’s left side ports (pictured in the top photo), which face our right side when we are working at an open laptop, the correct side ones to charge from? Thanks.




  • I tried to run my new HDXT Pro monitor from the right side USB ports on my new MacBook Pro 16 inch – the monitor would not turn on. However, it works fine if it is plugged into the left side ports. This monitor requires mixing of two data streams – so there may be additional right left differences in the ports. I worked on getting the monitor to turn on with a Senior Mac support specialist who decided it must be a faulty monitor. I decided to try plugging to the other side of the Mac Book Pro – and voila – no problem




  • Good tip. Makes sense. I’ll take your analysis at face value. Like you say, “The things you don’t know can hurt you!”




  • I wonder if this is true of using my 2015 15″ MacBook Pro.

    I recently noticed that kernel_task was going wild as the temperatures outside got higher (I don’t have AC). It couldn’t even run my 4K monitor (connected through a TB2 dock) without slowing to a crawl.

    I changed the thermal paste on the MBP last night and it now works fine with the 4K monitor on a hotter day. That said, when I start Google Chrome with the 4K monitor plugged in, kernel_task goes wild again. Without the 4K monitor, it runs Chrome just fine.

    I will try to see how it performs with Chrome + 4K monitor WITHOUT the power cable plugged in. Might be a way to get some desktop productivity with chrome without slowing to a crawl.




  • Don’t have this problem with my 2019 MBP 15 inch, though I also purchased one of those cpu coolers that’s a couple of fans that mounts under the computer (and as a benefit allows to to tilt the computer) because the computer tends to run hot and with the cooler the temp of the bottom of the MBP tends to run about 110-120 degrees.




    • 110-120 degrees of what? elephants? Sorry but I couldn’t resist the chuckle. Reminded me of my physics teacher who would scold anyone and everyone if they didn’t add units to their answer.

      For any other pedants out there, I’m assuming you mean 120 Fahrenheit which would be a very reasonable 58-59 Celsius.




  • What if the USB-C hub were plugged in on the right side? Would the power connection on the left side still be a problem? Is the problem really a left side issue, or is it only a problem when both are on the same side, either right or left?




  • I recently bought an OWC Docking station for my Macbook Pro. It came with a stout but short USB-C cable to connect to my machine and is closer to the left side, so I connected it there. Adding a USB memory stick will bring the fans on and my machine gets really hot. Now I know why! Thanks




  • I wonder what happens with no HDMI on the left. My HDMI adapters get really hot, but I do use a USB-b hub on the left for disks with no problems.




  • In this statement “if you’re charging your MBP using the left-hand ports AND have other accessories plugged into that side”, is the “AND” important? I normally have the power adapter plugged into one of the left side ports and two devices (TB3 Dock, monitor) plugged into the two right side ports – i.e. only 1 device on the left side.




  • This is it!
    I am using a 2018 15″ MBP.
    I took screenshots when the kernel_task goes up to 200% up… to send to Apple…

    So when do we use the left usb ports? :)




  • Meowrs! :)

    The kernel_task process is what appears when the machine has exceeded its thermal threshold and has to be throttled. It’s the kernel generating a null CPU task for the CPU cores that essentially locks out the CPU’s cores by using the kernel’s task prioritizing to create idle states as often as possible while still processing the instructions (non-idle states) from the software you’re using. Those idle states and the ramped up fans are what help cool the machine to go back under the thermal threshold as quickly as possible.

    It’s very likely that the path that the incoming power charge must take to the battery from the left port is either in direct line or proximal to other very high power components that are close enough to the TB3 proximity sensor that it causes that sensor to trigger the machine’s protection mode to cause the OS to throttle the CPU itself rather than wait for the built in CPU thermal throttling, which would be even more severe (and more dangerous as the rest of the machine would be much hotter in that case, which is very bad for components in such a cramped space.

    This actually seems like a really bad design oversight on Apple’s part more than anything else. They should have labeled one port specifically for charging the machine so users didn’t end up with poor performance due to thermodynamics.




  • Steve,

    I have one of the new 6-core 16″MBPs too, and it has a truly annoying “feature” I’d like you to do an article on.

    It regularly restarts itself; there is no pattern to it that I can see—and when it freezes (or restarts itself) all windows have to reload; password manager has to be restarted requiring a long password, etc., etc.—all this takes time. I have two other Macs (5K iMac and 2013 15″MBP) and neither ever need to restart in the middle of something. I am running latest Catalina on the offender, and earlier OSs on the other two.

    Is it Catalina, or something in the MBP itself? Any suggestions gratefully received; this is a real, time-wasting problem that has been there since the first time I powered it up.




    • Hi, Kit –

      Restarting itself seems indicative of an issue with the MBP. My 16-inch MBP doesn’t restart or freeze, and when I reboot — rarely — it loads quickly. Of course, it’s running the latest version of Catalina as well.

      That being said, I did — once — have a situation where the machine had tried to reboot itself in the middle of the night (I’m thinking that Apple pushed a security update) and was trying repeatedly to boot off of the Time Machine backup. When I started work the next morning, the machine was hot and the fans were blowing at full speed. Since then, it hasn’t repeated that behavior.

      One thing I can suggest is time-consuming and painful, but it may resolve the issue. That is to create a bootable backup, then boot from the backup, reformat the MBP’s drive, and do a clean install of Catalina. I had a MacBook Air that once did what you’re talking about, and that’s the only say I could resolve the problem.

      Steve




      • I’ve also encountered this issue with flaky RAM or a short in another hardware component that caused my laptop to lock up and occasionally reboot. I had an older laptop with removeable RAM. If the RAM is soldered to the motherboard, then hopefully the laptop is still under warranty. Most new laptops (Apple, Dell, HP) make their devices so thin that the what used to be modular is not anymore.

        I’d follow what Steve said by doing a clean install of Catalina. If it still acts up, make another backup and temporarily install Mojave or another High Sierra (which end-of-life might be this coming November 2020.) If the issue persists using other MacOS versions, then it might be hardware related and not OS related.

        Best wishes,
        Joe P




  • Is this unique to the left side? Or would you not see the same thing with the charging and peripherals plugged into the right side?




    • Hi, Jeff –

      The problem does appear to be specific to the left side, as if there’s an issue with either the temperature sensor used on the left side, a bad design that causes that side to get hotter than the right side, or in how macOS reacts to the high-temperature reading.

      The original post on StackExchange includes a set of tests in which the writer in which he swapped out power and USB-C cables. When everything was plugged into the left side (power and peripherals) the temp spiked and the fans kicked in, and kernel_task took over CPU usage. Plug power into the right and keep the peripherals on the left side, and everything was fine. However… (and this is from Adam on StackExchange):

      Note that high temperature on the right side appears to be ignored by the OS. Plugging everything into the two right ports instead of the left raised the Right temperatures to over 100 degrees, without the fans coming on. No kernel_task either, but the machine becomes unusable from something throttling.

      So it does appear that using one side for charging, the other for peripherals is the best practice in this case.




      • You guys think it would be valid to return this to Apple? I still have Applecare.




      • Steve,
        I’m using a tb3 hub (CalDigit TS3 plus) with all my drives and peripherals connected to it and it also handles the charging of my MBP all over one connection. I’m not sure how I would separate things and charge from the right and leave the peripherals on the left. There is no way to turn the charging feature off on the TS3 Plus. Should I just connect the TS3 Plus to one of the right side ports? What about the right side temps that are ignored by the OS?




        • Hi Kal,
          I think you will be fine using the Cal Digit that way, I am using one myself, and I was charging on the left side, I’ve switched over to the right side after reading this article.
          Because you aren’t using an additional USB-C port on your computer, you aren’t incurring the additional heat caused by more than one connection.




          • Julien,

            I do have a monitor connected to one of the right side ports. I spoke with Cal Digit and they told me to use my MBP charger on the right side and keep the TS3 Plus connected on the left side but use the opposite TB3 connections I use to connect the hub to my drives and computer. This way the TS3 Plus is not charging the MBP. Yeah I end up using a separate port to charge but it solved the problem I was experiencing and discussed in this article.