Work through these in order. Each one is cheaper than the one after it, and the answer is usually in the first three.
1. Is something actually busy? (ten seconds)
Activity Monitor, CPU tab, sort by CPU. Ignore kernel_task - it is the cooling mechanism rather than the cause, and we explained why in what kernel_task actually is. Look at what is second.
If something is sitting at 100% or more, that is your answer and everything below is irrelevant. Quit it and see whether the fans settle.
The usual suspects: a browser tab running something heavy, a sync client stuck in a retry loop, a stalled Spotlight index, an app that crashed but left a helper running.
2. Is it Spotlight, specifically? (one minute)
Worth its own step because it is common and self-resolving.
After a macOS update, a large file copy, or connecting a new external drive, mds and mds_stores reindex. They will use everything available and the machine will be hot and slow for anywhere between twenty minutes and a few hours.
If those processes are at the top of the list, the correct action is to leave it alone and come back later. Interrupting it means it starts again.
3. Where is it sitting, and is it charging? (thirty seconds)
Two environmental causes that produce identical symptoms to a software problem.
A laptop on a duvet, a cushion or a lap blocks the intake vents. On Intel machines this escalates within a couple of minutes. Move it to something hard and flat and wait five.
And on several Intel MacBook models, charging on the right-hand ports puts measurably more heat near the sensors than the left. If the problem appears when plugged in and disappears on battery, move the cable. It sounds like folklore and it is reproducible.
4. What happened just before? (needs a readout)
This is where Activity Monitor runs out. Thermal events are often over in ninety seconds, and by the time you have opened a window to investigate, the process that caused it has finished.
What answers it is a running readout you can glance at - the load in the minutes leading up to the fans, not the instant you happened to look. System Status Monitor keeps CPU, memory and load in the menu bar for exactly this.
The ordering is the whole diagnosis. If something else was busy first and heat followed, that process is the cause. If heat came first with nothing busy, the cause is environmental - vents, charging, ambient temperature.
5. Memory pressure, which looks like a CPU problem (one minute)
An underrated cause. When memory runs short, macOS compresses and then swaps to disk. Compression is CPU work, swapping is constant I/O, and both generate heat while the CPU graph does not look dramatic.
Check the Memory tab. Yellow or red pressure with swap growing through the day means the heat is a symptom of memory, not processing, and no amount of quitting CPU-heavy apps will fix it. We covered reading those two numbers in how much RAM a Mac actually needs.
6. Dust, if the machine is older than about three years
Last because it is the most invasive and the least likely on a newer machine.
Fans pull air through a heatsink, and over years the fins clog. The signature is specific: fans at full speed constantly, from boot, regardless of load, on a machine that used to be quiet. A clean install does nothing for this because it is not software.
On a desktop this is a can of compressed air. On a laptop it means opening the case, and if that is not something you want to do, it is the point to book a service.
What not to bother with
- Resetting the SMC. Occasionally relevant on older Intel Macs with confused sensors. Does nothing on Apple Silicon. It is the first suggestion in most threads and the least likely to apply.
- Fan control utilities. Running the fans harder lowers the temperature and does not address the load causing it. Treating the reading rather than the cause.
- Reinstalling macOS. Costs a day. Fixes a genuinely corrupted driver, which is rare, and does nothing about the browser tab that was the actual problem.
What is normal
Worth calibrating, because some heat is simply the machine working.
Fans audible during a video export, a long compile or a game is correct behaviour - the cooling system is doing its job. Warm to the touch under load is normal. What is not normal is fans at full speed while idle, sustained heat with nothing in the CPU list, or a machine that is hot from the moment it boots.
The first of those is nearly always a background process. The last is nearly always dust.