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User profile for user: MartinR
MartinR Author
User level: Level 6
19,157 points

iMac screen dims by itself (not set to auto adjust)

I have a 27" mid-2011 2.7GHz iMac, running High Sierra 10.13.2. Never had a problem with it until today.


Today the screen started dimming by itself. It is not set to automatically adjust brightness (never was).


After being on for a few minutes the screen dims by itself (brightness goes all the way down). In System Preferences > Displays I can manually adjust the brightness back up but a few seconds later the system turns the brightness back down again all by itself.


I tried restarting the system. I also tried turning it off & disconnecting from power, then later turning it back on. The behavior persists.


Any suggestions about what might be causing this? Ideas for further diagnostics? A solution?

iMac, macOS High Sierra (10.13.2), 27in/2.7GHz i5/8GB RAM/1TB+10TB

Posted on Oct 4, 2018 12:24 PM

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User profile for user: Tesserax
Tesserax
User level: Level 10
148,519 points

Oct 4, 2018 12:32 PM in response to MartinR

Please try the following, in order. Stop at any step where the issue is resolved:

  1. Reset your Mac's SMC.

    What: Reset the System Management Controller (SMC) on your Mac- Apple Support

    • How: Hold down the <shift><control><option> keys, and then, press the power button. Hold all of them for, at least, 10 seconds.
    • Why: Resets fans, keyboard backlight, status indicator, battery indicator, display backlight, power button, close/open the lid, sleep or shut down, battery doesn't charge, MagSafe indicator light incorrect status.
  2. Reset your Mac's NVRAM or PRAM.

    What: How to reset NVRAM or PRAM on your Mac - Apple Support

    • How: At boot-up, hold down the <option><command><P><R> keys.
    • Why: Resets sound volume, display resolution, startup-disk selection, time zone, & kernel panic information.
  3. Boot up your Mac in Safe Mode.

    What: Use safe mode to isolate issues with your Mac- Apple Support

    How: At boot-up, hold down the <shift> key until the Apple logo appears on the screen.

  4. Run the Apple Hardware Test (AHT) to uncover any hardware issues with your Mac.

    Use Apple Diagnostics or Apple Hardware Test- Apple Support

    About the "Apple Hardware Test does not support this machine" message- Apple Support

    How to Use Apple Hardware Test to Diagnose Mac Problems- OSXDaily

  5. Run an EtreCheck report and post it here so that we can help diagnose any issue that we may uncover with your Mac.

    You can download EtreCheck Pro fromhere or download it from the Apple App Store.

    Start EtreCheck from a normal user account. Optionally, you can run it from a user account with Administrator privileges for additional details.

    Select a problem from the drop-down menu to enable the "Start EtreCheck" button. Optionally, you can add comments on what issues your Mac is experiencing, especially to aide others with similar Mac issues.

    Click on Start EtreCheck

    Allow the program to run to completion.

    When done, select Report from the left-side window to display it.

    • Select the "Share Report" icon.

      Select Copy Report

    • Paste the report to your reply post. (Note: The EtreCheck report does NOT contain any personal information.)

    Ref:Using EtreCheck- ASC (etresoft)

  6. Make a Genius Bar appointment at your local Apple Store.

    Check Your Service and Support Coverage- Apple Support

    For getting the right support, here's a genius idea- Apple

    To contactApple Supportdirectly.

    You can only make 1-week ahead appointments either via the web link above or AppleCare phone support. The appointment timeframe is longer if you go in-person to an Apple Store.

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User profile for user: MartinR
MartinR Author
User level: Level 6
19,157 points

Oct 5, 2018 3:33 PM in response to MartinR

I did some more investigating.


First, I discovered that on the model iMac I have, the SMC is reset automatically on startup provided the power was previously disconnected. So that got done.


Second, after shutting down the system for an hour or so, it came back on fine for about 15-20 minutes, then the screen started doing its dimming again. Got me thinking it might be a thermal problem somewhere. Well, a little more searching on the internet turned up some cases where the display did funny things when the system got hot ... and there were specific references to checking the ventilation holes on the bottom of the iMac as well as the vent strip on the top. Lo and behold, there was an awful lot of dust clogging over half of the vent holes on my iMac! So I vacuumed everything out, clean as a whistle.


Third, I ran Apple Hardware Test and it found no problems.


By this point I had done 3 or 4 more cold restarts. After closing down from AHT the system booted up fine, I adjusted the display brightness back to where I wanted it and I haven't had a problem since. Today I had the iMac running for about 8 hours without any problems.


Who would have thought dust in the vents would have caused this? I'm glad it seems to have been that simple a fix.

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iMac screen dims by itself (not set to auto adjust)

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