Reset and maintain your SMC and NVRAM

Most of your Mac is operated by software which is loaded from your startup volume. But in order to boot it up sufficiently to be able to load that up, and to enable certain hardware features controlling the cooling fans, battery charging, etc., every Mac has firmware and persistent storage for some key settings. These include the EFI firmware, the SMC, NVRAM and (in certain models) the T2 chip.


Extensible Firmware Interface (EFI)

EFI firmware is specific to the model of Mac, and may differ according to which version of macOS you have installed. To understand it better, read this guide. Lists of firmware versions for different versions of macOS, and further help, are:

System Management Controller (SMC)

In Macs without a T2 chip, the SMC is a custom chipset with its own firmware which may sometimes be updated separately from the EFI. This controls many of the extended hardware features like cooling fans. Aside from ensuring that its firmware is kept up to date (as for EFI firmware), the SMC may on occasion need to be reset when one or more of its functions has gone awry, such as the fans are stuck on full blast all the time. Apple explains these in its article on the SMC.

Features which are controlled by the SMC include:

  • the cooling fans; if these seem stuck full on even when your Mac is just idling, then that is suggestive of an SMC problem.
  • accessory lights, such as keyboard backlighting, status indicator light, battery indicator lights on non-removable batteries, those around the I/O ports on recent Mac Pros.
  • power button; if this is not a mechanical problem (older models), suspect the SMC.
  • opening and closing the lid of a notebook.
  • sleeping and shutdown; if your Mac sleeps or shuts down when you don’t think it should, suspect the SMC.
  • battery charging (laptops), and the MagSafe power adaptor light.
  • napping and processor slowing; if your Mac performs very poorly but the CPU is not showing high load, suspect the SMC.
  • Target display mode; if this does not work properly, or it switches unexpectedly, suspect the SMC.
  • recognition of peripherals connected to a USB-C port; if your MacBook or MacBook Pro doesn’t recognise these, suspect the SMC.
  • the Sudden Motion Sensor (SMS), ambient light sensing, and selection of an external video source for some iMacs.

Mac models which include a T2 chip – all current models apart from the iMac – incorporate SMC functions within that chip., and are detailed below.

How you reset the SMC depends on the Mac, with different routines for laptops with and without a removable battery, and for desktops.

To reset the SMC in a laptop with a non-removable battery (without a T2 chip):

  1. Shut the Mac down.
  2. Connect its mains adaptor and ensure that power is being supplied to it.
  3. On the built-in keyboard, hold down Shift-Control-Option on the left side and press the Power button with them held down for 10 seconds.
  4. release all the keys, wait a few seconds, then power up normally (preferably to reset the NVRAM next).

To reset the SMC in a laptop with a user-removable battery (older models):

  1. Shut the Mac down.
  2. Disconnect any mains adaptor.
  3. Remove the battery.
  4. Press and hold the power button for 5 seconds.
  5. Insert the battery, then connect the mains power adaptor and ensure that power is being supplied to it.
  6. Power up normally (preferably to reset the NVRAM next).

To reset the SMC in a desktop (without a T2 chip):

  1. Shut the Mac down.
  2. Optionally disconnect all peripherals apart from any Ethernet cable, keyboard, mouse/trackpad, and any display. For the procedure to be most reliable, connect any wireless keyboard using its USB charging cable. Then disconnect the mains power cable from the back of the Mac.
  3. Wait for 15 seconds, then reconnect the mains cable.
  4. If you are going on to further system diagnostics or testing, leave peripherals disconnected. Otherwise you can reconnect the peripherals that you need.
  5. Wait for 5 seconds, then power up normally (preferably to reset the NVRAM next).

Although you do not have to reset the NVRAM straight after resetting the SMC, because of potential overlap between their functions and problems, it is usually wise to do so.

Disconnecting peripherals is an additional measure which is not included in the standard SMC reset procedure. I recommend it because you may well want to have those peripherals clear when you are trying to sort this type of problem out. If you are sure that you will just proceed to reset the SMC (and possibly the NVRAM), you do not need to do that.

T2 chip

Macs with a T2 chip incorporate the SMC, image signal processor, audio controller, SSD controller, and the secure enclave processor in the single SoC (system on a chip), which behaves quite differently from regular SMC chips, although signs of malfunction remain the same as those listed above.

To reset the SMC functions in your Mac’s T2 chip:

  • Shut the Mac down.
  • Once shut down, press and hold its Power button for 10 seconds.
  • After releasing the Power button, wait a few seconds, then press the Power button to start the Mac up.

If that doesn’t work, try one of two fallback procedures for Macs with T2 chips.

For an iMac Pro or Mac mini:

  • Shut the Mac down.
  • Disconnect its mains power lead for 15 seconds.
  • Reconnect its mains power lead, and wait for 5 seconds.
  • Press the Power button to start it up.

For a MacBook Pro or MacBook Air:

  • Shut the Mac down.
  • Once it is shut down, press and hold the right Shift key, the left Option key, and the left Control key for 7 seconds. Then keep holding those keys while you also press and hold the Power button for another 7 seconds. During this, the Mac may turn on and off again.
  • Release those keys and the Power button, and wait a few seconds before pressing the Power button to start it up again.

These procedures are detailed here.

Non-Volatile Random-Access Memory (NVRAM)

NVRAM, formerly Parameter Random-Access Memory or PRAM, stores some key settings which your Mac cannot obtain from disk during startup. Apple explains it in a separate article about NVRAM.

The NVRAM contains settings for:

  • sound output volume,
  • display resolution, and sometimes other display settings,
  • startup volume (disk),
  • any recent kernel panic, to enable a kernel panic log to be constructed,
  • third-party kernel extension policy (High Sierra only),
  • other model-specific features.

If you are curious, you can use the shell command
nvram -xp or nvram -p
to list all the variables which are stored in your Mac’s NVRAM. The output will be formatted slightly differently according to model, and you may find it best to try both, to work out which is more meaningful.

In an iMac17,1, they include:

  • Bluetooth internal controller information,
  • the display configuration,
  • the computer name,
  • EFI recovery information,
  • System audio volume DB,
  • Bluetooth active controller information,
  • System audio output volume,
  • Backlight level,
  • Whether Location Services are enabled,
  • EFI Bluetooth delay setting,
  • Details of the last run of Apple Hardware Test.

When running High Sierra or later, they should also include the current third-party kernel extension policy.

To reset the NVRAM on any model of Mac:

  1. If your Mac has a firmware password set, you must turn that off before attempting to reset the NVRAM, or the procedure will fail. To do that, restart in Recovery mode and select the Firmware Password Utility or Startup Security Utility from the Utilities menu.
  2. If your Mac is running, shut it down. For the procedure to be most reliable, connect any wireless keyboard using its USB charging cable.
  3. Position your fingers on the Command, Option, P and R keys ready to press and hold them down.
  4. Power your Mac up using its Power button as normal.
  5. Immediately after pressing the Power button, press and hold those four keys. If you’re worried about doing this quickly enough, you can hold the four keys down as you press the Power button on most models.
  6. If your Mac plays a startup chime (chord), hold those four keys until your Mac sounds the startup chime a second time, when you should release them.
  7. If your Mac doesn’t play a startup chime (chord), hold those keys until the display backlight turns on a second time, or the Apple logo appears and disappears for the second time.
  8. Release the keys. If you do not release them now, your Mac will restart again, and may reset the NVRAM a second time.
  9. Allow your Mac to start up normally.

As of this writing, Apple’s description of this process isn’t quite correct for recent models and recent firmware, as it only refers to the startup chime (no longer supported in any current model) and the appearance of the Apple logo, which will never happen if you follow the procedure it describes.

To understand this sequence correctly on your recent model, observe a normal startup sequence. This begins with the display being completely black and unlit (normal off mode). The first change seen in the display is when its backlight (or equivalent) turns on, although the display itself remains black (early boot). After that, the white Apple logo appears, and the boot sequence progresses from there.

When resetting NVRAM on one of these recent models, continue holding the four keys when the display backlight turns on, and after a short interval it will turn off again, leaving the display in normal off mode. Keep holding the fours keys until the display backlight turns on a second time, then release them, and normal startup should continue from there.

If you continue to hold the four keys down after the second time that the display backlight turns on, the backlight will turn off, and the NVRAM will be reset a second time. There’s no harm in that, but that second reset is unnecessary. This is easier if your Mac is older and doesn’t go through the early boot phase, and simpler still when it still has a startup chime. However, as long as your Mac’s display isn’t in very bright light (it shouldn’t be anyway), it’s easy to identify when its backlight first turns on, and to reset the NVRAM reliably.

If you want to be certain that you have reset the NVRAM properly, the most reliable way is to compare its contents before and after resetting, using the command
nvram -xp or nvram -p
Following successful reset, the NVRAM contains a lot less information in fewer fields.

Remember that when you reset the NVRAM, all the above settings will revert to their default. If you have changed the third-party kernel extension policy using spctl, then you will need to repeat that change of setting.

One simple way to reset both the SMC and NVRAM without keyboard gymnastics is to start up in Single-user Recovery mode using Command-S-R, then enter the two commands
pmset -a restoredefaults
nvram -c

before restarting with reboot. This should also be possible when a hardware password is set. (Thanks to Manoli for suggesting this.)

Resetting the SMC and NVRAM are not panaceas for every problem. They will not fix disk problems, nor failing graphics cards. Used appropriately, they can transform a Mac which seems to be dying into one that is in fine fettle.

Information about how to use the NVRAM to change the behaviour of your Mac is here.