Part 1 of 3 — Safe Mode and a New User Account
Start by restarting the Mac in Safe Mode and testing whether the brightness and volume adjustments still behave erratically.
Apple's instructions explain how to start an Apple silicon Mac in Safe Mode.
If the problem persists, restart normally and test again. Occasionally, starting in Safe Mode and then restarting normally may resolve certain software-related issues.
If the problem continues, create a temporary user account using Apple's instructions: Set up users, guests, and groups on Mac.
Log out of the main account, log into the new account, and repeat the tests.
The results may help narrow down the cause:
- If the problem occurs only in the original account, a user-specific setting, login item, or background utility may be involved.
- If the problem persists in Safe Mode and in the new account, a system-wide software issue becomes more plausible, although further investigation may still be necessary.
For reference, Apple's official guide to the relevant controls is Adjust brightness and volume on your Mac.
Part 2 of 3 — Examine the System Configuration
If the preceding tests do not identify the cause, download EtreCheck directly from its developer.
EtreCheck is a diagnostic utility that provides an overview of the Mac's hardware, software, startup items, background services, and other system configuration details. It is not a definitive diagnostic tool, but its report may help identify software that could be contributing to the problem.
The application is available in free and paid versions, with additional features in the paid version.
After running EtreCheck, use its Share Report option to copy the report and post it using the Additional Text option in your reply.
Before posting, review the report and redact any personal or identifying information you do not wish to share publicly.
Please include the following information alongside the report:
- Exact macOS version and build number.
- Mac model and chip type.
- Whether the problem occurs in Safe Mode.
- Whether the problem occurs in a newly created user account.
- Whether the keyboard controls, System Settings sliders, and Control Center sliders are all affected.
Part 3 of 3 — Clean Setup as a Last Resort
If the problem persists after the preceding tests, and there is reason to suspect a system-wide software issue, a clean setup may help determine whether the behaviour persists without third-party software or restored user settings.
This is an advanced troubleshooting step and should not be undertaken without a verified backup.
For an Apple silicon Mac, Apple's Erase your Mac and reset it to factory settings guide explains how to use Erase All Content and Settings to remove user data, applications, and settings while retaining the currently installed operating system.
Before proceeding, make a Time Machine backup and verify that important files can be recovered.
If you decide to proceed:
- Erase only one Mac initially and Do Not Restore from a Backup - Full Stop
- Set it up as a new Mac without restoring from a backup or installing third-party applications.
- Test the brightness and volume controls under normal use.
- Record whether the original behaviour returns.
If the problem persists on a clean system, that would strengthen the case for a system-level issue, although it would not conclusively establish the cause.
If the problem disappears, reintroduce applications and settings cautiously to see whether the behaviour returns.
There is no need to erase both MacBooks unless the results from the first test justify doing so.
Additional information
Because the same behaviour occurs on two different MacBooks and affects both keyboard controls and on-screen sliders, it would also be reasonable to contact Apple Support if the issue persists after these tests.
When reporting the issue, provide the exact macOS build numbers and the results of the troubleshooting steps above. This may help establish whether the problem is associated with a particular software configuration or a broader system issue.
One small technical distinction worth retaining: Apple's brightness and volume guide describes the normal adjustment controls; it does not confirm that the erratic behaviour you've described is a known macOS bug. The troubleshooting results will be more useful than assuming a cause before the tests are complete.
