2015 Macbook Pro display goes blank but machine continues to run and fans continue to go even if I close lid I have a 2015 Macbook Pro that seems to turn the display off after it reaches a specific temperature.  Is this a reasonable conclusion?  

Initially I thought I had to change the logic board completely and figured it was full on hardware failure.  I tried closing opening the lid, tried messing with the display dimming, tried resetting SMC and PRAM, tried posting on Apple and asking them.  Nothing worked.  So rather than pull the whole thing apart and find a replacement logic board and replacing that I bought a new macbook pro 2018 and gave up on the 2015.

I needed some pictures off the old machine and began looking for a way to get the data off the 2015.  I started looking for a converter to transfer the data off the proprietary SSD.  I thought I would try to start the machine up again after about 6 months and the display came up asked me to log in and I did, everything was working fine for about two minutes then screen just goes black, machine still runs just no display.  Hard shut down then restarted worked fine for about 30 secs then blank screen.  Hard shut down left it overnight and then came back and restarted next day worked fine for about 3 mins. enough time to get my files  to the cloud and get the stuff I needed.  But it seems that all my hardware is working fine up to a point.  I've been able to run it sometimes up to about 10 mins, even updated the OS but eventually the display will just turn itself off and I have to repeat the process of hard shut down letting cool itself then restart.

So my questions are: Should I pull the heat sync and put some new paste on the cpu and gpu and see if that will help?  Has anyone else encountered this issue?  Any other thoughts on what might be happening here?  Any other idea about how I can cool the machine better or if I should not even mess with trying to address this issue?  I love my old 2015 and would love to have it working again.  Any suggestions would be helpful.  Thanks.
You've already reset the System Management Controller and NVRAM (both logical first steps), but there's no mention of you testing the MBP in Safe Mode or of testing your hardware. 

Next I would probably start with testing to see if this behaviour is replicated when booted into Safe Mode and if the issue persists then running Apple Diagnostics.

Boot into Safe Mode 

Follow these steps to boot your Mac into Safe Mode: 


Fully shut down your MBP 
Restart your MBP
Immediately press the Shift key and keep it down 
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). 
Take a note of what happens (i.e. does your MBP still behave the same way, etc) 
Exit Safe Mode by restarting your MBP as normal 
Test again when booted normally 


If the problem doesn't occur while booted into Safe Mode, let me know and we'll go from there. 

If the problem persists, proceed with running Apple Diagnostics.

Run Apple Diagnostics

Follow the steps below to run Apple Diagnostics:


Fully shut down your MBP
Restart your MBP
Immediately press the D key and keep it down until you see the Diagnostics screen appear
Wait for Diagnostics to finish (this typically only takes a few minutes)
Once complete, one of two things will appear on the screen:


a No issues found message
a brief description of any errors found plus further instructions

If the diagnostics test does find errors, take a note of what they are


Note: 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. 

Regardless, take a note of what happens and let me know how you went.