Power your MacBook Pro from the right side USB-C Ports, Here's why.

Tuesday, April 28, 2020 5:24 PM

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

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 Proximitytemperature, which is caused by charging and having normal peripherals plugged in at the same time.

Oddly enough, someone else ran into this problem a few months ago while reviewing a USB-C webcam. Not knowing about this issue, They 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.