I have an early 2009 Mac Pro (2 x 2.66 GHz Quad Core) that recently started having issues. It started as the black screen issue, that I troubleshooted to find that there was an issue with the CPU tray. I found this out by doing the usual troubleshooting, replacing the graphics card and then buying a used second machine and interchanging the CPU cards between chassis.

I ended up replacing the 2009 Octocore CPU tray with an Early 2009 2.8GHz Quad core tray, as this machine has now become a spare machine, but the fans are cranking loudly.

When I run hardware diagnostics I see that the PS, Exhaust and Intake fans are not running and the PCI and Boosta fans are running at 850 rpm - seemingly not too far above min spec.

I would love to get guidance as to how to get the three fans running again and got my lovely mac pro back to its usual quiet self. The loud noise is unbearable in my home office.

Thanks for the guidance.

  • So, the fans are running top speed but they're not being reported as running? The boosters generally run at 1800-2500 at a reasonable idle (fast but still quiet). The larger case fans tend to top out around 3000, which is like living near an airport ;) Nominally, I'd expect boosters at 2k, case fans at 1k, which is essentially 'silent'. I'd suspect a sensor if it was one fan misbehaving, but 3 or more… I think you have a motherboard issue (which I'm not qualified to diagnose further, I'm afraid). Maybe have a look at iStat Menus who I think do a freetrial.
    – Tetsujin
    Dec 12, 2020 at 16:41
  • They're showing as zero on the mac fan control app. What do you mean by motherboard? The spine? I already changed the CPU tray.
    – Wellardmac
    Dec 12, 2020 at 16:50
  • I think you have too many sensors not reporting for it to really be a sensor issue per se. I don't know precisely what they all report back to, but that would be my suspect. Data is being sent, but not received.
    – Tetsujin
    Dec 12, 2020 at 16:52
  • This only started as we were troubleshooting the black screen issue. It was silent before, but initiated after many reboots PRAM and NVRAM resets. Prior to that the machine was silent .
    – Wellardmac
    Dec 12, 2020 at 16:54
  • You changed hardware - did you reset SMC? Shut down, unplug mains, wait 15s, press power button 5s, release button, plug back to mains, wait 10s, boot. (This is a kind of 'composite' reset, which definitely works on all Mac Pros) … Ohhh… & change the "CMOS" battery. CR2032 coin battery, maybe a quid/buck/euro from any supermarket ( or pence on eBay if you buy a lot)
    – Tetsujin
    Dec 12, 2020 at 16:59

1 Answer 1


After some research I discovered that the fans were blowing because I put a single CPU tray into a machine that originally had a dual CPU tray. Apparently it doesn't like this mismatch and runs the fans full, as it thinks it has suffered failure of a CPU.

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