MacBook Pro fan’s are running at high speed continually (even when CPU usage is low)

I had an issue with an Apple MacBook Pro that was constantly running the fans at high RPM’s. Sometimes the CPU was running high (90-100%) without justification (simply browsing the web and checking email) other times the CPU was running low (10-15%). Steps to resolve included:

1. Reset the NVRAM (non-volatile random access memory)
2. Reset the SMC (system management controller)

How to reset NVRAM on your Mac¹

Follow these steps to reset your Mac computer’s NVRAM.

If you have a late-2016 MacBook Pro, the steps are slightly different. Skip to the next section for instructions.

1. Shut down your Mac.
2. Find Command (⌘), Option, P, and R on your keyboard.
3. Turn on your Mac.
4. Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
5. Hold these keys until the computer restarts and you hear the startup sound for a second time.
6. Release the keys.

After resetting NVRAM, you might need to re-configure settings for speaker volume, screen resolution, startup disk selection, and time zone information.

If issues related to these features continue on a desktop Mac (like an iMac, Mac mini or Mac Pro), its logic board battery might need to be replaced. The logic board battery on desktop computers helps retain NVRAM settings when your Mac is unplugged. You can take your Mac to an Apple Store or Apple Authorized Service Provider to replace the battery on the logic board.

Reset the System Management Controller (SMC) on your Mac²

Before resetting the SMC

Reset the SMC only after you’ve tried all other standard troubleshooting. Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to see if the issue still occurs.

1. Press Command-Option-Escape to force quit any application that is not responding.
2. Put your Mac to sleep by choosing Sleep from the Apple menu. Wake the computer after it has gone to sleep.
3. Restart your Mac by choosing Restart from the Apple menu.
4. Shut down your Mac by choosing Shut Down from the Apple menu.
5. If your Mac isn’t responding, force the Mac to shut down by pressing and holding the power button for 10 seconds. You’ll lose any unsaved work in any open applications.

If you’re using a Mac notebook computer that is having issues related to power or the battery:

1. Unplug the power adapter from your Mac and the electrical outlet for several seconds, then plug it back in.
2. Shut down your Mac. 
3. Remove and re-insert the battery, if it’s removable battery.
4. Restart your Mac.

If the issue still isn’t resolved, you might need to reset the SMC. See the indicators and steps below. 

Indicators that your SMC might need to be reset

After you perform normal troubleshooting, these symptoms may indicate that an SMC reset could be necessary:

• The computer fans run at high speed, though the computer is not experiencing heavy usage and is properly ventilated.
• The keyboard backlight behaves incorrectly on Mac computers that have this feature.
• The status indicator light (SIL) behaves incorrectly on Mac computers that have this feature.
• Battery indicator lights, if present, behave incorrectly on Mac notebooks that have a non-removable battery.
• The display backlight doesn’t respond correctly to ambient light changes on Mac computers that have this feature.
• The computer doesn’t respond to the power button when pressed.
• A Mac notebook computer doesn’t respond properly when you close or open the lid.
• The computer sleeps or shuts down unexpectedly.
• The battery doesn’t charge properly.
• The MagSafe power adapter LED doesn’t indicate the correct activity.
• The computer is performing unusually slowly, though it isn’t experiencing abnormally high CPU utilization.
• Application icons may bounce in the Dock for an extended amount of time when opened.
• Applications may not function correctly, or they may stop responding after being opened.
• A computer that supports target display mode does not switch into or out of target display mode as expected, or it switches into or out of target display mode at unexpected times.
• The illumination around the I/O ports on a Mac Pro (Late 2013) does not activate when you move the computer.

Reset the SMC on Mac notebook computers

First determine whether the battery is removable. Mac notebook computers that have a non-removable battery include MacBook Pro (Early 2009) and later, all models of MacBook Air, MacBook (Late 2009), and MacBook (Retina, 12-inch, Early 2015). Learn more about Mac notebook batteries.

If the battery is non-removable 

1. Shut down the Mac.
2. Plug in the MagSafe or USB-C power adapter to a power source and to your Mac.
3. Using the built-in keyboard, press Shift-Control-Option on the left side of the keyboard, then press the power button at the same time.
4. Release all keys, then press the power button again to turn on your Mac

If the battery is removable

1. Shut down the Mac.
2. Disconnect the MagSafe power adapter from the Mac.
3. Remove the battery. (Learn about removing the battery in MacBook and MacBook Pro computers.)
4. Press and hold the power button for 5 seconds.
5. Reconnect the battery and MagSafe power adapter.
6. Press the power button to turn on the Mac.

The LED on MagSafe power adapters might change states or temporarily turn off when you reset the SMC.

Reset the SMC on Mac desktop computers

Follow these steps for iMac, Mac mini, Mac Pro, and Xserve.

1. Shut down the Mac.
2. Unplug the power cord.
3. Wait 15 seconds.
4. Plug the power cord back in.
5. Wait 5 seconds, then press the power button to turn on the Mac.

For Intel-based Xserve computers that aren’t responding, you can shut down locally or by using remote commands. You can also shut down by pressing and holding the power button for 5 seconds.

Learn more

Resetting the SMC does not reset or otherwise change the contents of NVRAM or PRAM on Intel-based Macs.

The SMC is responsible for these and other low-level functions on Intel-based Macs:

• Responding to presses of the power button
• Responding to the display lid opening and closing on portable Macs
• Battery management
• Thermal management
• The SMS (Sudden Motion Sensor)
• Ambient light sensing
• Keyboard backlighting
• Status indicator light (SIL) management
• Battery status indicator lights
• Selecting an external (instead of internal) video source for some iMac displays

If neither of these steps fix the fan issue, you might have a more serious hardware related issue.

References:

¹ https://support.apple.com/en-us/HT204063

² https://support.apple.com/en-us/HT201295