Carsten Vollrath

I/O Board Error LED #1: Flex Check: This indicates that there is a connection issue between the logic board and the I/O board. Check the mezzanine connectors on both the logic board and I/O board to be sure the connections are fully seated. I/O Board Status LEDs #2: 11V Power: This indicates 11V power from the power supply. It should remain illuminated whenever AC power is connected. #3: 12V Power: This indicates 12V main system power. It should illuminate once the power button has been pressed, and should remain on while the system is powered on and not in sleep mode. If high-power USB or Thunderbolt devices are connected prior to sleep, this LED will remain illuminated during sleep. #4: Platform Reset Release: This indicates that the system has transitioned to a fully running power state (S0). During startup, this indicates that the system can transition to EFI Boot. #5: S5 Power State: This indicates that the S5 (soft power off) state is available. It will illuminate at all power states, until the...

Logic Board Error LEDs CPU Check: This indicates that there is a connection issue between the logic board and the CPU riser card. Verify that the edge connector on the CPU riser card is fully seated into the slot on the logic board. GFXA Check: This indicates that there is a connection issue between the logic board and Graphics Board A. Check the mezzanine connectors on both the logic board and graphics board to be sure the connections are fully seated. GFXB Check: This indicates that there is a connection issue between the logic board and Graphics Board B. Check the mezzanine connectors on both the logic board and graphics board to be sure the connections are fully seated.

Sounds like your I/O board has gone bad. Its pretty common actually.

[belge: mp-2013-late.pdf] Here is the Repair manual. Hope this helps

No need for that. Your Mac Pro has Diagnostic LEDs on the I/O board. The 2nd one from the top will tell you if the PSU is supplying 11V.

The Mac won’t come on without the housing. You will need a magnet. The 2 LEDs that come on when pressing the button only verify that it’s plugged into a power source and getting 12V. Remove the bottom cover, then you will be able to access all diagnostic LEDs. Photo shows what kind of magnet will work and where to hold it to power the Mac on without the housing.