Late 2015 27", GPU temps resets iMac

[pbedrosi]

In certain apps my GPU temps climb to high 70 deg C and shutdown/reset the iMac.

I have been using Macs Fan Control to monitor temps and noticed only temp rising above all have been the GPU. But fan speed stays constant in auto mode around 1,300 RPM.

1) Watching YouTube videos - Fix has been to set "ambient mode" off in video settings
2) Recent...Adobe Acrobat pro - reducing file size
3) Scanner - Open scanner operation under printer, have HP printe/scanner. Scanning document - I can watch the GPU temps climb.
4) Full Screen Streaming - ESPN+

I can manually get around Scanner/Acrobat by manually maxing our fan speed in Macs Fan Control. My question is why the fan is not automatically rising accordingly with GPU speed?

My machine has a used logic board, upgraded i7 CPU and SSD's.

 

[pbedrosi]

I suppose basic question is why is my fan stuck in low and not adjust with rising temps

 

[Nguyen Duc Hieu]

I suppose basic question is why is my fan stuck in low and not adjust with rising temps

Perhaps.
I also use Mac Fan Control, and I often set it to accelerate speed when the GPU reach 45 Degree Celcius, max at 60 degrees.

 

[pbedrosi]

I'm using Macs Fan Control, I have fan speeds set based on GPU temps "GPU AMD Radeon R9 M390". I'm wondering if this is b/c of thermal pads on the memory chips? even on this site with the video ads the temps rise. I was just on IKEA site building a self designed shelving the fans would adjust up but eventually after 5 min the iMac shut down.

I would hate to open it back up again...I hate the whole heat sink removal and thermal paste process...