External monitor not working over Thunderbolt after upgrade to Big Sur 11.6 on two M1 Macs

Question

External monitor not working over Thunderbolt after upgrade to Big Sur 11.6 on two M1 Macs

Hi Mac Community,

After upgrading my M1 macs (24" iMac and 13" MBP), neither works over Thunderbolt 3 to a monitor that previously worked without a hitch using the exact same setup. The monitor *does* still work over HDMI using a dongle.

I've done the steps at these support links:

https://support.apple.com/en-us/HT201177

https://www.lifewire.com/fix-external-display-not-detected-on-mac-5096070

(including the Display preferences, scaling, adjusting resolution, rebooting, unplugging and replugging the cable, restoring from sleep, etc.)

I have not reset the NVRAM or the SMC, as I am not sure how to do them on the 24" iMac (no wired keyboard), and frankly, I'm not sure why I would need to when the Thunderbolt configuration was working perfectly prior to the upgrade to 11.6.

Any feedback or ideas? Thanks in advance!

iMac Line (2012 and Later)

Posted on Sep 14, 2021 8:46 AM

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Answer 1

Top-ranking reply

Jonathan Terry1 Author

Level 1

13 points

Sep 14, 2021 6:13 PM in response to Jonathan Terry1

Quick update:

The monitor continued to work fine on all Macs running 11.5, but not on the 11.6 ones. It is a Samsung M7 (not a Dell or Apple monitor). So, I can't blame the cable or the monitor, as it works with other Macs, and it worked with my iMac and MBP prior to the 11.6 upgrade.

Soft resets and the above steps didn't resolve the issue.

Oddly, when I unplugged the monitor for a more hard reset, and plugged it back in after 30 seconds....it worked.

I have no explanation for this other than perhaps 11.6 changing something in the firmware for the USB-C / Thunderbolt ports to communicate differently for their video signals.

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Answer 2

tbirdvet

Level 6

14,471 points

Sep 15, 2021 5:18 AM in response to Jonathan Terry1

I updated to 11.6 on my Mac Mini M1 using TB3 to monitor and no issue. However, the printer queue will not close after printing leaving the printer icon in the dock (others have also reported this issue).

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Answer 3

rkaufmann87

Level 10

143,982 points

Sep 14, 2021 8:52 AM in response to Jonathan Terry1

Please disconnect all wired peripherals including the external display and then restart in Safe Mode ( How to use safe mode on your Mac - Apple Support ) and then restart normally and re-test.

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