Thermal throttling

Watch for a heavy task that starts fast and then slows down, with fans running loudly on Macs that have fans. A Mac that feels hot to the touch while doing it is another sign.

In Terminal, pmset -g therm prints any thermal warning levels macOS has recorded. What it reports depends on the Mac model.

Use the Mac on a hard, flat surface and keep its vents clear. Soft surfaces such as a bed or a lap can block airflow. Avoid leaving it in direct sun or a hot car.

Quit apps that keep the CPU or GPU busy in the background. If a Mac runs hot while idle, check Activity Monitor for a process that is stuck using CPU.

macOS keeps a record of thermal events that affect CPU speed. These commands only read that information.

1. Run pmset -g therm. If nothing has happened, it prints notes such as No thermal warning level has been recorded. On many Intel Macs it also prints a CPU_Speed_Limit value, where 100 means no limit and a lower number means macOS is capping CPU speed.
2. Run pmset -g thermlog and start the heavy task. It keeps printing thermal notifications as they happen. Press Control-C to stop it.
3. Run sudo powermetrics --samplers thermal -n 1 for the current thermal pressure level. Nominal means there is no thermal pressure. It asks for your administrator password.
4. In Activity Monitor, watch whether kernel_task starts using a large share of CPU during the task. Apple describes this as a way macOS keeps processor time away from busy apps while the CPU is too hot.

• Long, heavy work such as video exports, 3D rendering, games or large builds.
• A background process stuck at high % CPU or % GPU, so the Mac is working hard while it looks idle.
• Blocked vents, or using the Mac on a bed, a couch, a pillow or your lap. Apple says fans may run more on soft surfaces.
• A hot room. Apple notes that when the room is hot, the fans turn on sooner and run faster.
• Direct sun or a hot car, which heat the Mac before it even starts working.

MacBook Air models with Apple silicon have a fanless design. They are silent, but they cannot speed up a fan when they get hot. During a long heavy task, macOS keeps the temperature in range by lowering performance instead.

Short tasks are rarely affected. If you often run long exports or builds, expect them to take longer on a fanless Mac than on a Mac with a fan and the same chip.

Choose Apple menu > System Settings > Battery, then look at Energy Mode. Low Power Mode reduces energy use, and Apple says it can also reduce fan noise on Macs that have fans. The trade off is lower performance.

Some MacBook Pro, Mac mini and Mac Studio models also offer High Power Mode. Apple says it lets the fans run at higher speeds, which may allow higher performance in very intensive workloads, with more fan noise.

If the fans run fast when the Mac is not busy and the vents are clear, Apple suggests restarting a Mac with Apple silicon, or resetting the SMC on an Intel Mac.