Diagnostics LED's meaning in a Mac Pro 2013

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 computer is fully shut down. If the computer is scheduled to turn on at a certain time in the Energy Saver preference pane, this LED will still illuminate after the computer has been shut down.

#6: S0 Power State: This indicates that the S0 (full power) state is available. It will illuminate once the computer begins the startup sequence, but does not necessarily indicate that computer has successfully started up.

#7: S4 Power State: This indicates that the S4 (hibernate) state is available. It should illuminate at the same time as the S0 LED. If, during startup, this LED illuminates without S0 illuminating, it may indicate an issue.

#8: PCIe Switch Status: This indicates the status of the link from the PCIe switch on the I/O Board to the CPU. During startup, this indicates that the system has reached EFI Boot. If the link to the PCIe switch is not operating in PCIe v3 mode, this LED will blink. A fast blink indicates v2 mode, while a slow blink indicates v1 mode. These states may correlate to issues with Thunderbolt or USB devices.

—————————————————————————————————————————-

CPU Riser Card Warning LEDs

CPU_PROCHOT: This indicates that an overtemp condition has been detected by the CPU or SMC. It may also be triggered by the total system power exceeding its specified limit. Use MRI or ASD to further isolate the issue.

MEM_EVENT: This indicates that an overtemp condition has been detected in a DIMM. Use MRI or ASD to further isolate the issue.

CPU Riser Card Error LEDs

CPU_CATERR: This indicates that the CPU has reported a catastrophic error. The system software will be unresponsive in this state. This may be caused by an unexpected power off of components connected to the CPU, or an error in EFI during startup.

CPU_ERROR: This indicates that the CPU has encountered a fatal error. The system software is unresponsive.

—————————————————————————————————————————-

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.