With much of Europe wilting in the heat this week, it’s time to remind ourselves of the warning signs that our Macs are getting too hot. As Macs don’t sweat in the heat as we do, it’s also vital to understand what we can do to help cool them down.

One of the most serious warning signs of imminent heat stroke in humans appears paradoxical, as they stop sweating, and their skin may become pale rather than red. This is because their circulation has collapsed, and if they aren’t cooled very quickly, they’re at real risk of serious injury or death.

Hot Macs have their own paradox: open Activity Monitor, select the CPU view, and at the top of the CPU % list will be kernel_task hogging the CPU cores with 100% or more, rather than its usual 4% or so. Kerb any temptation to kill it and hope it goes away, as it has taken control of your Mac to let it cool.

Causes

Every Mac contain multiple temperature sensors that are monitored by its Core Duet sub-system, enabling its System Management Controller (SMC) to manage its internal environment and services. When a process runs away and takes over the cores, or internal thermal sensors register abnormally high temperatures, Core Duet responds with a set of actions to increase cooling. These include its active cooling system of fans, which run up to speed to blow cooling air over sensitive components, and reducing CPU load to lower the production of heat. Where possible that will slow processor speed, and block any runaway processes by occupying the CPU with kernel_task.

Whatever you do, don’t try to stop kernel_task from doing its job. That’s a bit like stopping someone from drinking to replace their fluid loss in sweat, and will only make matters worse. When the fans get loud and kernel_task hogs your CPU, it’s time to do all you can to cool your Mac down.

There are rare occasions when fans blow full on and kernel_task goes wild without any thermal problem. In Intel Macs, resetting the SMC is usually curative, but this could instead be the result of a fault in a thermal sensor, or in the SMC. Hardware diagnostics should tell you more. By far the most common cause of persistent problems is dust and debris in the air ducts, and in the case of some Intel Mac notebooks overheating of the left USB-C ports. For more details, see Apple’s note about kernel_task, which also confirms what I have written here.

Warning signs

One of the best early warning signs of overheating, even if your Mac doesn’t have audible cooling fans, is sluggish handling, as a result of kernel_task taking over the CPU cores. Scrolling windows with video and images becomes jerky, response to controls is delayed, and switching between apps takes noticeable time. These can result from any tasks that are using excess CPU, and it only takes a few seconds to check in Activity Monitor what’s going on.

You can then progressively quit open apps that might be contributing to the overheating, keeping a watch on kernel_task in the CPU view. This can also help you identify which apps are most responsible, and could be safely opened again once your Mac has cooled. This can be most useful for Intel Macs, with their greater heat production.

Actions

If your Mac is showing early signs of thermal strain and still running, encourage heat loss by immediately:

• opening its lid fully if it’s in clamshell mode,
• ensuring all ventilation openings in its case are free of obstruction,
• clearing space around the whole of its case, including the underside, to aid convective cooling,
• if the Mac is in sunlight, moving it into the shade,
• if you have an external fan, turn that on to cool its case by forced convection,
• if air conditioning is available, move the Mac to a cooler area.

Reduce its heat production by immediately:

• quitting all open apps, if necessary force-quitting any that won’t cooperate,
• killing all non-essential processes that are taking more than 5-10% CPU in Activity Monitor except kernel_task,
• disconnecting any mains charging cable, if there is sufficient charge in its battery.

If a Mac has cooling fans, it’s better to leave the Mac running, as shutting it down will also stop those fans from blowing cooling air inside it. Some like to put cold packs against the case, which can cool the outside more quickly than an external cooling fan. If you wish to try that, ensure that condensation doesn’t enter the Mac, particularly through its keyboard or ventilation openings, and don’t obstruct either the keyboard or those cooling ducts.

High and Low Power Modes

Some Macs offer one or two energy modes in their Battery or Energy settings. Where available, these can be useful for controlling a Mac’s heat production and loss, but they work quite differently. Their labelling may also appear confusing: these are offered as the Energy Mode, but its menu choices should refer to High Power Mode and Low Power Mode. Apple provides detailed information on these, and which models support each mode.

Low Power Mode reduces heat production by limiting the frequencies of the CPU and GPU cores, so maximum heat production is lower, and the battery will run longer on a charge. This also allows any cooling fans to run quieter.

High Power Mode doesn’t do the opposite and increase heat production from normal. Instead, the Mac uses its fans more aggressively to keep it cooler for longer when undertaking long and demanding work.

When you’re trying to cool your Mac down because it has become too hot, you should find Automatic or High Power best, once you’ve closed all apps, although Core Duet should take charge and do what’s most effective at the time.

Persistent problems

In the most extreme heat, if you can’t use your Mac where there’s air conditioning and kernel_task keeps running at high CPU %, you should get your Mac as cool as possible, to the point where kernel_task is taking below 10%, then you may be able to continue working with just one app open at a time. If that doesn’t work out, you’re best leaving your Mac to idle and cool, then shutting it down, to leave it until temperatures have fallen.

Stay cool!