I have a MBP 2010 which I reliably connected (for many years) to an external LG 25UM64-S UltraWide monitor at 2580×1080 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?
Best Answer
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.