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The Mac Pro First Generation is an Intel Xeon-based workstation computer manufactured by Apple Inc. The first generation model includes the machines from 2006 through 2008.

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Will this machine power on with only one CPU?

I recently acquired a 2006 Mac Pro, which, according to the diagnostic LEDs on the logic board, has a fault on CPU B. I've tried replacing the thermal paste and checking all connections, but to no avail.

I've also tried removing the second CPU entirely, but this still doesn't work. The machine powers on and the fans spin up (the GPU fan in particular is rather loud), but nothing happens beyond that.

It may also be worth noting that when both CPUs are installed, the system will push out a video signal, though it's just a black screen. It appears to be restarting endlessly and encountering the same error. When only one CPU is installed no video signal is sent out.

Am I missing something, or can the Mac Pro only boot with both CPUs installed (I've even tried putting the heatsink back and plugging in the temp sensor)?

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Have you tried reinstalling the OS? Something could be corrupt and preventing the system from starting properly.

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There's no hard drive in the system right now, but I can't even get far enough to attempt that if there was one. If something is corrupted it's within the computer's EFI, which can't be fixed with a simple OS reinstall.

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One of my 2008 Pros shipped with only one CPU. I believe all the 2006-2008 Pros shipped with two sockets even when only one CPU was present. I did do an upgrade from a single to dual on my Mac Pro "Quad Core" 2.8 (2008). I'm just thinking out loud here.

There was a fan syncing software program on the original system installation disk and it had to be used with any major tear down or you got run away fans. I would try booting from the original system installation disk. Let us know your results. Also that CPU should be dirt cheap.

On my 2008 I went from a 2.8 GHz Q. Core Xeon E5462, to a 3.0 GHz Q. Core Xeon E5472 x2 for under $75 and most of that expense was the additional heat sink.

Let us know what happens with the optical drive boot. You are going to have to have that original installation disk in any case. It can probably be found on eBay.

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Will do. I haven't found an original install disc but I've ordered a pair of Xeon 5160 CPUs. I've been trying other troubleshooting steps like resetting the SMC and PRAM (I can't get far enough to reset PRAM with the keyboard so I removed the CMOS battery), swapping out the video card/trying different slots, etc.

I can verify USB ports are receiving power, as the light on the mouse is lit when plugged in.

I've also tried booting the Firmware Restoration CD, but I can't even get far enough to do that (the system doesn't respond even after holding the power button as instructed).

One more thing, I've been looking at the diagnostic LEDs on the logic board while booting up. The LEDs for trickle power, GPU present and power on are lit as they should be, but LED 8 (EFI good) never comes on except for when the power button is first pressed, after which it immediately goes out and stays that way. If I'm not mistaken, the LED should light after a successful POST (when EFI is ready to hand control over to the OS).

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Just got the new set of CPUs in. After installing them both, I powered up the machine. The power LED comes on and the fans spin, and I even get a faint chime. But the video signal is still blank and the chime repeats infinitely. The LED for a fault on CPU B is still lit. I tried resetting firmware; the power LED flashes and the long tone sounds, but the disc drive doesn't open and eventually the Mac just goes back to repeating the chime. Resetting SMC and PRAM have had no effect.

I tried one CPU again and got the same result as previously described: light on, fans spinning, but no video.

I tried both CPUs again but swapped, and still got the same CPU fault LED. This leads me to believe the second socket may be toast.

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