I think
@h9826790 also said that Macsfancontrol software can override this problem, but I can't find it now and I might be remembering it wrong.
Your memory is correct. I did say that, and it still work in the latest MacOS.
There are basically 2 types of fan control software.
1) Altering the minimum fan speed
2) Altering the target fan speed
Most of the software use method 1.
e.g. Your mac commanding an idle 600RPM, and the software alter the minimum RPM to 800. So now the fan can only run at 800RPM but not anything lower.
The good thing by using this method is the system protection still there.
e.g. Your mac now commanding a target 1000RPM because something is hot, since it's above the altered minimum 800 RPM, the fan will run at 1000RPM eventually.
However, MacsFanControl use method 2.
That means if your mac commanding an idle 600RPM, the minimum still 600RPM, doesn't change. However, the software now commanding a target 800RPM. Therefore, the final fan speed is 800RPM.
And if your Mac commanding a 1000RPM because something is hot, MacsfanControl will override this number and make it 800RPM. So the fan will still only run at 800RPM, and ignore the system setting (or protection).
For something like CPU, I personally won't recommend use MacsFanControl to override the system fan profile, because the lost of protection (of course I mean if the user set a wrong fan profile, e.g. a fixed low RPM, which can overheat the CPU).
However, since the PCIe fan is not directly controlling anything's temperature (most graphic card has it's own fan), therefore, it won't hurt anything to alter the target fan speed.
With this SMC bug, the system may command a target speed 1500RPM (or even higher) PCIe fan after boot. So, most of the fan control software can't do anything about it. But by installing the MacsFanControl and with the following setting.
The PCIe fan now will base on the PCIe ambient temperature, but nothing else. In the above setting, the PCIe fan will stay at idle RPM until the PCIe ambient hit 40C, and rise 100PRM for every 1C. Unless the PCIe ambient is high after boot (which should not happen in general), MacsFanControl will command a target 800RPM, which will override the buggy 1500RPM system setting. (of course, that 40C can be changed. This is just my own setting and works fine)
Therefore, by installing MacsFanControl, set it up correctly, and allow it to auto start after boot. The system now has a fully automatic solution to fix that high PCIe fan speed after boot. And the PCIe fan will also spin up when something warm up the PCIe compartment. So, the PCIe fan's duty now is to make sure the air temperature in the PCIe compartment is cool enough for the graphic card's own fan can work effectively.
