I updated to big sur my fan won't stop running

You can do an SMC Reset to get everything working together again.

How to reset the SMC of your Mac - Apple Support

also, are you running any Third-party Virus scanners?

are you running any third-party file SYNC programs such as DropBox, OneDrive, or similar?

I had the same issue that lasted for weeks after I updated to Big Sur. Last night I decided to try and track the issue down as best as I could. I ended up finding that GoogleChromeHelper was hogging an insane amount of CPU when I looked at my Activity Monitor. I'm not sure if this is related to the update but as soon as I killed that, updated chrome, and changed the privacy settings in chrome to disable plugin access (GoogleChromeHelper 'helps' with external plugin access) my fan stopped running and GoogleChromeHelper stopped hogging so much CPU.

Hope this helps.

Had the same problem. Shutting the computer down and starting it again looks to have solved it.

take a look at what it's doing with Activity Monitor (a built-in App in the /Applications/Utilities folder).

View CPU activity in Activity Monitor on Mac - Apple Support

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This is normal after a large macOS software update. What it is likely doing is reindexing Spotlight which can take a long time, and should stop after a day or so.

Are you sure this is normal? The plug into the wall is getting hot with my fan running all day. My Mac is hot -- and it's just a year old. Worried about a fire hazard.

I have the same issue and it is now almost a week since the upgrade. The CPU is running about 85% - 90% idle. Any hints?