Diagnosing a 2009 Mac Pro - red LED, no boot

[eric2006]

Hey guys,
I'm working on fixing a 2009 Mac Pro (single CPU). It currently spins up and the power light is solid, but does not chime or have video output. It has a red LED on near the CPU heatsink (on the CPU board), and all diagnostic LEDs light up (when the button is pressed). I've tried replacing the CPU with no result. The power supply has a good fuse. USB devices don't seem to get any power (caps lock key presses etc). Tried different RAM etc.
From what I've read, this could be the power supply, logic board OR the CPU/RAM board. Any ideas on narrowing this down? My friend also has a 2009 Mac Pro but I'm hesitant to try any parts on that, the dual CPU board would be very expensive to replace. Thanks

 

[Cheryl]

No sound or video but the hard drive is spinning - sounds like the mother board.

 

[eric2006]

Yeah, I would replace the logic board but I am a bit confused with this machine as the processor has its own board and that is where the error LED is lighting up. I did some additional searching and found that the light can be on on that board even if the error is on the main board. I think I will see if I can test the CPU board in my friend's computer to completely isolate the logic board.

 

[eric2006]

Turned out to be the main logic board, not the CPU board. The USB power thing was a red herring, I don't see the caps lock light until I am booted into the OS.

 

[wenorder]

Turned out to be the main logic board, not the CPU board. The USB power thing was a red herring, I don't see the caps lock light until I am booted into the OS.

I know this is a really old thread, but this sounds like the same issue I am having with my 2008 mac pro. Could I ask how tricky it was for you to replace the logic board?