Chosen Solution
I have a Mid 2011 iMac Model: MD063LL/A that will not allow me to adjust screen brightness, I’ve tried using the function keys on the keyboard, as well as in system preferences, the bar moves as if the screen is adjusting but the screen remains at full brightness. The screen two days ago was at low brightness and would not let me turn it up, and after a reboot, and waiting a couple hours it magically came back to full brightness. Any idea what can be the cause of this? Computer is fully updated as well.
Lets start off trying a SMC reset. Follow this Apple T/N: Reset the System Management Controller (SMC) on your Mac. Let us know if that fixed things. Update (02/03/2017) Well you have me scratching my head here a bit ;-} I’m wondering if your NVRAM battery is having issues so the setting in NVRAM is not remembered or corrupted do to the power loss. To test this you’ll need to go to the clock settings. Open Date & Time preferences control panel. What we want to do here is alter how your clock works. Unlock the panel and then uncheck the ‘Set date and time automatically’. This will force the system to use its built-in clock to keep time (which uses the battery). Save the settings then shutdown your system and unplug the AC cord for a good hour (more if you can). Now plug AC cord back in and power up the system is the clocks date & time correct? If not the battery is failing! If that doesn’t nail it I suspect your logic board is having problems which will require bringing the system into Apple or Apple authorized service center.
I’m having a similar issue on my mid-2011 iMac. Had to upgrade to Catalina but I no longer have brightness control…I suspect this update is the culprit. Even after resetting SMC und PRAM, neither System Preferences > Displays show any brightness control setting, nor in the upper dashboard.
F1 and F2 keys on keyboard don’t work - tried with both a plug in USB keyboard and a bluetooth apple keyboard. Trying Dan’s suggestion of date/time reset and unplug. Any other ideas? UPDATE Only solution I found was to download this app https://apps.apple.com/us/app/brightness…
