Hello all, This is regarding a known issue with Mac Pro 4,1 and 5,1's, where installing certain video cards (usually non-Apple cards, but not always) results in improper system fan control. There are several threads from over the past year or so, making it hard to pick just one to respond to. I decided to start a fresh thread, but would encourage anyone who's interested in the topic to check out these previous discussions as well: http://forums.macrumors.com/showthread.php?t=1566964 http://forums.macrumors.com/showthread.php?t=1671758 http://forums.macrumors.com/showthread.php?t=1648459 (particularly interesting toward the end) http://forums.macrumors.com/showthread.php?t=1649650 (perhaps the most comprehensive) New 1/11/14: http://forums.macrumors.com/showthread.php?t=1694103 (gathering SMC data from the community) Background I wanted to revisit this topic, to share some of my own observations, and see if there's been any new headway made toward understanding the problem. I recently purchased a new 2012 6-core model (built June 2013), an upgrade from my previous 2008 2.8 8-core (3,1). Among the components that were migrated to the new machine is an EVGA GTX 670 (with no EFI). I never had any unusual fan issues in the 3,1 but in the new 5,1, I've experienced fan behavior that's consistent with what others have encountered: the PCI and PS fans run well above their normal levels, and can vary wildly based on different factors. There are several variables that others have observed in the past that may affect this behavior: Whether it's an OEM Apple card (though some have reported problems using the Apple 5770 in 4,1's) Whether the video card has EFI (though some have reported problems with recent Mac Editions from EVGA and Sapphire as well) PCIe slot 1 vs. 2 Drive configuration (SSDs vs HDDs and different combinations) For single-power cards, cable A vs. cable B (coinciding with slots 1 and 2) Cold starts vs. soft restarts/sleep Some folks also wondered if it was only 4,1's (or 4,1's with the 5,1 firmware hack). However, my machine is a brand new 2013-built 5,1, so that doesn't seem to be the case. There have also been some recent fan problems that are specific to Mavericks, which may or may not be related. My Observations My system has two SSDs in sleds 1 and 2. These are the main Mac and Windows boot drives. I have access to three video cards for this machine: the OEM Radeon 5770, an older OEM 5870, and the GTX 670. I did a variety of tests, and can provide specific RPM numbers if it's helpful, but I'll just stick to a summary for now. In general, the Apple cards (Radeon 5770 and 5870) both behaved in the way one would expect, with low system fan speeds at idle and no unusual ramping up under load. There are a few minor differences between the two, but nothing out of the ordinary. The Radeon 5770 was also fine using either power cable. With the GTX 670 installed, however, I observed the following: In slot 1, the PCI and PS fans run high after a cold start, but can be "fixed" with the graphics load trick (running the OpenGL Extensions Viewer benchmark). After this, the system fans settle down and seem to behave exactly as they should. This well-mannered behavior carries over through soft restarts (including into Windows) and through sleep/wake. A cold start reverts to bad behavior. In slot 2, the PCI and PS fans run slightly high, but closer to normal. The OpenGL Extensions Viewer trick has no further effect. One interesting oddity: installing a HDD in sled 3 drops the PS fan speed to normal (600rpm), but the PCI remains the same. Overall, having the video card in slot 2 would be acceptable for me if not for the fact that the fans still ramp up unnaturally high when the video card is under moderate to heavy load in Windows. Others have said that using slot 2 works around the problem, but in my case at least, it's only a partial workaround. A few other notes: - Booting from a HDD made no difference - Booting from a fresh installation of Mountain Lion made no difference - I have yet to try Mavericks Conclusion So for me, the only way to get 100% correct behavior is to use slot 1 and the OpenGL Extensions Viewer trick after cold starts. The fans then run at their correct 800/600/600/600/856 speeds at low-load, and only increase slightly under heavy load. Based on all my observations, I'm inclined to think the problem is more about an error in power-sensing than heat-sensing. It actually makes me wonder about something I noticed when I first set this machine up. I ran the hardware test a few times, and it kept producing an error message related to a Power Supply sensor. After researching the error, I found that it was documented on the Apple support site, where they claim it's essentially a false positive: http://support.apple.com/kb/TS4051 It may be entirely unrelated, but it does make me wonder. In any case, I hope this post is helpful. If anyone has any questions or other information they can share, please do. Thanks!