iMac monitor going to full brightness

Question

Level 1

5 points

iMac monitor going to full brightness

When I connect an external monitor and change the resolution to scaled on both monitors for larger icons etc, the iMac screen will go to full brightness on its own, whether or not the brightness is set to manual control or automatic. The change will occur; after I unlock the screen, after the screen saver turns on or the screen goes to sleep. The change will also occur if I'm simply working for a while. I have already reset the NVRAM and SMC as per the instructions here: https://support.apple.com/en-us/HT201177. I believe there is a bug in the software when having two screens and scaling them. The problem does not occur when I unplug the second monitor power and signal cable and run a scaled screen with the iMac only. Here are the details of my set up:

21.5-inch Late 2015 iMac

macOS: Catalina 10.15.1

Monitor scaling setting: 1600x900 for both monitors

2nd monitor: Hewlett Packard 22er

Signal cable with built-in adapter: Thunderbolt from iMac to HDMI at 22er

iMac 21.5", macOS 10.15

Posted on Dec 4, 2019 9:14 AM

Reply

Answer 1

den.thed

Level 10

91,612 points

Dec 4, 2019 10:27 AM in response to JJsurf092

Go to: Apple > System Preferences > Displays > select Arrangement > uncheck Mirror Displays.

Then for other tips and tricks, have a look at > Use external monitors with your Mac - Apple Support

It is also beneficial to uncheck some or all of the boxes in Mission Control until you get a feel for Dual Displays.

Reply

Answer 2

rkaufmann87

Level 10

148,494 points

Dec 4, 2019 12:25 PM in response to JJsurf092

I would recommend testing another external display, this could be an issue with the external display you are using. I recommend this as this is the first time (in many years of working on this forum) that I have seen this issue.

Reply

Answer 3

rkaufmann87

Level 10

148,494 points

Dec 4, 2019 10:08 AM in response to JJsurf092

Please do the following:

Reset the NVRAM per the directions in https://support.apple.com/en-us/HT204063
Restart in Safe Mode per the directions in https://support.apple.com/en-us/HT201262 and then restart normally and re-test.

Reply

Answer 4

JJsurf092 Author

Level 1

5 points

Dec 4, 2019 10:25 AM in response to rkaufmann87

This didn't work, but thanks for the suggestion. Ironically, after I reset the NVRAM and started in safe mode, the screen was at full brightness, as it usually is when I boot up. I rebooted normally and still have the problem.

Reply

Answer 5

JJsurf092 Author

Level 1

5 points

Dec 4, 2019 10:53 AM in response to rkaufmann87

This may have actually worked after all. At the moment, I'm not having the issue. Maybe I just needed to reboot another time after implementing your suggestions. If I have the problem again, I'll post again. Thanks for your help.

Reply