I've long wondered why, without the help of Macs Fan Control (MFC henceforward), the fans in my Mac Pro Early 2009 8 core never speed up. This goes as far back as Mountain Lion, which came on it when I bought it (used) several years ago. (I can't speak to any earlier versions of MacOS.)
I also wonder if one of the temperature sensors in my Mac Pro is bad.
The IOH Diode sensor reads very high in this system. Without MFC speeding up the fans, it used to read around 220 F. Before becoming aware of this I ran it that without any fan boost for at least a year and a half.
220 F is hot enough that I'd think it would fry the diode right off, yet it kept on working for a long time at that indicated temperature. Hence my question about a possibly faulty sensor.
Just to be safe, though, since I bought MFC I've adjusted it to boost all the fans except the PCI and power supply, when the IOH Diode temperature starts to read really high.
Since then the system has kept on chugging along, fairly quietly, with the IOH Diode sensor indicating around 205 most of the time.