I have a MBP 2010 which I reliably connected (for many years) to an external LG 25UM64-S UltraWide monitor at 2580x1080 native resolution using a mDP<->DP cable. The 25" monitor broke and I replaced it with a newer, bigger LG 29UM68-P UltraWide monitor, also with DisplayPort.

When I connect my MBP, the monitor detects the connection, but reports "No Signal" when I switch the input to DisplayPort. The SAME monitor+cable works fine with a newer MacBook Air.

I have updated macOS to the latest version of Sierra (10.12.6). When I open the Display applet, it detects the LG Monitor, but reports it as a 34" model. I can hit "Gather Windows" and change the resolution on the LG, but I always get a gray screen with "No Signal"

update: I have tried to reset the SMC once, also the NVRAM (as per below) and even checked the monitor while booting into safe mode. None of these steps changee the outcome. What else can I try?

  • Does the newer MBA with same cable work with the 25" monitor? I am wondering if the monitor that "died" was actually the MBP.
    – Allan
    Commented Oct 30, 2017 at 4:45
  • the 25" monitor was smashed in a move. ;-( The 29" (used) monitor was an insurance replacement...
    – michael
    Commented Oct 30, 2017 at 13:04
  • Got it. That definitely fits the description of "broken monitor." That said, have you tried cycling the power of the monitor making sure the DP input is selected?
    – Allan
    Commented Oct 30, 2017 at 13:10
  • yes. the monitor works fine when connected to a newer macbook air.
    – michael
    Commented Oct 30, 2017 at 14:44
  • Power cycle when connected to the older MBP.
    – Allan
    Commented Oct 30, 2017 at 14:57

2 Answers 2


Here's what worked for me (none of the above worked): unplug the DP cable. Then slowly plug it in until the laptop screen (not the external) flickers and returns back to the normal image, then immediately push (not too hard) the cable in the rest of the way. Seems that some MacBooks have finicky ports.


I note you've already reset the SMC, but there are a few other things you can try as well.

Reset the NVRAM

To reset the Non-Volatile Random-Access Memory (NVRAM) on your model of MBP, follow these steps:

  1. Shut down your MBP (i.e. a full shut down, not just logging out).
  2. Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
  3. Hold those keys down until your MBP reboots again (i.e. a 2nd time) and you here the startup chime.
  4. Let go of the keys and let your MBP reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

If you find this makes no difference, you can also test to see what happens when booted into Safe Mode.

Boot into Safe Mode

Follow these steps to boot your MBP into Safe Mode:

  1. Fully shut down your MBP
  2. Restart your MBP
  3. Immediately press the Shift key and keep it down
  4. Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
  5. Take a note of what happens (i.e. try connecting your display to determine if the issue is still present, etc)
  6. Exit Safe Mode by restarting your Mac normally and try connecting your display again

If your display connects during Safe Mode but doesn't connect afterwards when booted normally, let me know. Otherwise, if you had no joy regardless of whether you were in Safe Mode or not, it's probably time to test your hardware.

Run Apple Hardware Test

Your model MBP uses Apple Hardware Test. To use this, follow these steps:

  1. Shut down your MBP
  2. Restart your MBP
  3. Press and hold the D key before the gray startup screen appears.
  4. After a while, Apple Hardware Test (AHT) will start.
  5. When prompted, select your language and click the right arrow.
  6. When the AHT console appears, you can choose to run Basic tests by clicking the Test button. However, I suggest you select the "Perform extended testing" checkbox before you click the Test button.
  7. Your test results will appear in the window in the bottom-right of the console.

Note 1: that the extended test will take some time. Take a note of the results and report back.

Note 2: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.

  • tried NVRAM and safe mode, both did not fix the problem. will have to fine time to try the hardware test. i'm hoping I can find another ext monitor with displayport to check my MBP, but no luck yet.
    – michael
    Commented Oct 30, 2017 at 14:47

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