Hi all this one is weird. I have a Mac Pro 5,1 with dual processor tray. When There is one processor installed, it boots fine, I use software to slow down the fans and all is well. When I have two processors installed, together with heatsinks and memory , it boots fine PROVIDED I leave it in the freezer for twenty minutes and then install the tray and boot it before it warms up again. When it warms up it crashes. If I leave out cooling it in the freezer it fails to boot. Have checked the circuitry on the heatsink sensor and that is in order. The fan software gives a temperature (heatsink) for processor A of 21 C and processor B of 27 C before the system crashes. I suspect it may be a faulty heatsink fan (B) but cannot tell when it is installed in the chassis. The fact it boots at all indicates there's no problem with the processor(s) or memory or tray circuitry. Anybody can tell me how to test the fan when its outside the system? Apply 5 volts? where? Anybody got any other ideas about how to resolve this temperature issue? I'm running Snow Leopard or Yosemite depending on what I'm doing. Thanks in advance, you guys are the business.