The iMac’s fan is quite silent; at its default speed of 1200 rpm, I don’t hear it. The only times it revs up is when I’m using Skype for videoconferencing, or when I rip a video with Handbrake. I have a late 2014 5K iMac with the AMD Radeon R9 M295X graphics card.
But in recent days, I’d noticed that my iMac’s fan was running more often than usual. Since I already use iStat Menus, I set up the app to display sensors in a menu. The menu displays temperatures and voltages for more than a dozen locations in the Mac.
At the time, I noticed that the GPU Die was showing a temperature of more than 90 C, even when the Mac wasn’t doing anything. You can see the spikes early in the Last 7 Days graph, when it was over 100 C when viewing a video. It reached as much as 105 C, which is not necessarily a problem, but the fact that it didn’t go below about 90 C was suspicious. (Note that the peaks in the last hour occurred when I was running a GPU benchmark app, which pushed the graphics card to the limit.)
I couldn’t find any conclusive information about this on the internet, so I contraction Apple. The technician checked and said that it certainly wasn’t a normal temperature. He was prepared to set up a repair, but first had me reset the SMC then reset the NVRAM on the iMac.
To my surprise, this resolved the issue. The resting GPU Die temperature dropped to the 50s-60s C range, and the fan no longer runs fast when I’m not taxing the computer.
So if you encounter this issue, try the above steps: reset the SMC, then the NVRAM. Please post a comment if this does resolve the problem, or if it doesn’t.