MacBook Air 2018 keyboard backlight works only with FileVault

Because the keyboard backlight works at the FileVault password screen and then immediately stops when macOS loads, a DFU firmware restore is a worthwhile next diagnostic step. 

You have already performed multiple clean macOS installations, so another normal macOS reinstall is unlikely to provide much additional information.

Your 2018 MacBook Air has the T2 Security Chip, and Apple's DFU procedure can Revive or Restore its firmware.

This is different from reinstalling macOS.

First, make a current backup.

Then use a second Mac connected to the Internet and a USB-C data cable. Shut down the affected MacBook Air and connect it to the second Mac.

To enter DFU mode, press and release the power button, then immediately hold Left Control + Left Option + Right Shift + Power/Touch ID for about 3 seconds. The MacBook Air's screen should remain blank. The second Mac should display the Mac in DFU mode in Finder.

Start with Revive Mac. A Revive reinstalls the firmware/recovery components without erasing your data. 

If that does not correct the problem, the next step is Restore Mac. A Restore erases the Mac and restores its firmware and operating system, so make sure your backup is current.

After a DFU Restore, set the Mac up temporarily as a new Mac rather than immediately restoring your backup. 

Then test the keyboard backlight, F5/F6 keys, and Keyboard Brightness control before installing additional software or restoring your data.

This makes DFU Restore a particularly useful diagnostic in your case. 

If the backlight works normally after a DFU Restore on a completely clean system, that would strongly indicate that the previous problem was in the macOS/firmware software path. 

If the backlight still works only in the FileVault pre-boot environment and disappears as soon as the clean macOS installation loads, that would shift the evidence substantially toward a hardware, T2, SMC, or hardware-to-firmware communication problem.

It cannot by itself prove that a particular hardware component has failed, but it would be a much stronger test than repeatedly reinstalling macOS.

Apple's official instructions are here: How to revive or restore Mac firmware.

The troubleshooting already performed is useful. Clean installations of Monterey, Ventura, and Sonoma, the successful Apple diagnostics, and especially the observation that the keyboard backlight works at the FileVault password screen provide important clues.

The missing keyboard-backlight controls in Keyboard settings, the F5/F6 keys not responding, and the Control Center slider returning to zero indicate that macOS is not successfully controlling the backlight. These symptoms don't by themselves identify the cause.

The fact that the backlight works before the FileVault password is entered shows that the physical backlight can illuminate and that at least part of its control path is functioning.

The fact that it turns off when macOS takes over is therefore particularly useful, although it doesn't completely exclude a hardware-related problem.

Testing several macOS versions makes a problem specific to one release less likely, while passing Apple's diagnostics provides additional evidence that no readily detectable hardware fault is present.

Neither, however, completely rules out a firmware, sensor, controller, or hardware/software interaction.

Before replacing parts, the remaining useful checks would be to reset the SMC, test the keyboard backlight in macOS Recovery, and verify that the ambient-light sensor is functioning normally. These tests could help determine whether the problem occurs at the hardware/firmware level or only after normal macOS takes control.

Overall, the existing evidence points to a problem with the backlight's control path during the transition from the FileVault/pre-boot environment to normal macOS, rather than establishing that the backlight hardware itself has failed.

From what you describe, it seems there is likely to be a hardware issue within this Mac. Since it is a 2018 MacBook Air, it has the T2 chip with Secure Enclave. The Secure Enclave is used to to protect and handle encryption keys, and a hardware encryption engine, another hardware element, is utilized to perform encryption and decryption at very high speed, which is essential for performance.

I think this is hardware because you did clean installs of myriad versions of the macOS and all show the issue. There seems to be cross talk between the integrated circuits and/or chips associated with the Secure Enclave (or possibly the hardware encryption/decryption engine) and the backlighting circuitry. Such cross talk should never be seen, so

there is a malfunctioning circuit or chip allowing signals to interfere with another circuit. Possibly shielding has degraded and no longer works. You could even consider this to be a form of RFI, which was not there when the laptop was new, so now something has physically changed and the signals are interfering.

I think this would be almost impossible to track down and repair. The parts may or may not be available anymore, but a definitive diagnosis is daunting and would require detailed schematics and special expertise with probes and tiny voltages and currents. Unless you like an adventure and scientific experiment, the cost would far exceed the value of the laptop and maybe even the cost of a current brand new laptop.

What I would do is accept the limitations of an aging laptop which can certainly continue to be used as it clearly is still functioning. But (I think, and I certainly could be wrong not having even seen the device in person) this is hardware degrading so I would be attentive to doing frequent backups as one doesn't know what might be next. And it might continue working fine (minus the backlight) as is for years into the future.

It could be that if you open up the laptop, you might find broken or degraded insulation and that might indeed be repairable.

gsm0786 wrote:

Things I have tried so far:

Reinstall macOS

Wipe and install macOS 12

Wipe and install macOS 13

Wipe and install macOS 14

When you did these "wipe and install macOS" steps, did you wipe the laptop following this:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

And then did you test BEFORE migrating anything from the old system back to the "wiped" laptop?

If you migrated and indeed this was due to software, a configuration, or something installed (as Owl-53 and HWTech have suggested might have happened), then you should test the "wiped" system -- "wiped" as indicated in Apple's link above) -- BEFORE migrating items back.

Otherwise you could be bringing back the cause of the issue.

I will backtrack a bit on my earlier post suggesting a hardware cause if you did not follow the procedure specified by Apple above and tested before anything was migrated back or installed. Because then a software or configuration cause has not yet been ruled out.

I would suspect an issue with some third party software on your system. You can try booting into Safe Mode to see if that makes any difference....if the keyboard backlight works fine in Safe Mode, then likely this confirms an issue with some third party software. However, it is possible that third party software modified a configuration file which is unaffected by Safe Mode.

I think there is Problem in Mac OS and apple should fix it, if hardware problem in my device then it shows the keyboard brightness in "system settings" but no any setting did not show in "system settings"

There is Mac OS problem.

Any Configuration file location which could help in this, please share.

Ignore