One of my 2015 Mac Book Pro’s Thunderbolt2 ports was detecting my monitor, but not sending it a video signal.
I tried all the usual suggestions to fix this: reset NVRAM, reset SMC…
But the issue turned out to be caused by a bent pin on the bottom of the Thunderbolt 2 port. I had to look into the port with glasses and a bright light to see it. The pin was gold/brass in color, and was bent up from the bottom against the inner back wall of the port.
The pin didn’t look like it was one of the ones that is supposed to come into contact with the Thunderbolt circuitry, it looked instead like a pin intended to keep the connector in place with some small upward pressure.
The fact that it was bent up against the back wall was keeping the connector from seating completely and maybe preventing full contact of all port circuitry, hence the “partial” functionality: monitor detected, but monitor not receiving signal.
I fixed the issue by inserting a very narrow, metal pick and picked/pulled the pin off the back wall and bent it down into place again. The pin didn’t break, and the cable went in completely this time, and the monitor came back to life.