Mac Pro 6,1 Late 2013 Diagnostic LEDs

The Mac Pro (Late 2013) contains several diagnostic LEDs and test points to assist in troubleshooting:
Status LEDs are green, and do not illuminate unless the DIAG button (refer to the I/O Board LEDs topic) on the I/O board is being pressed. This type of LED is only found on the I/O board. They are used to identify power states and are useful in troubleshooting the startup sequence. 
Error LEDs are red, and illuminate continuously when there is a problem. 
Warning LEDs are yellow, and illuminate continuously when a thermal problem is detected. This type of LED is only found on the CPU riser card.
Using Diagnostic LEDs for Troubleshooting The Mac Pro (Late 2013) contains a Hall Effect sensor that will prevent operation of the computer with the housing removed. For the purpose of viewing diagnostic LEDs only, it is possible to briefly operate the computer with the housing removed. 
From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off). Remove the AC power cord and all cables and peripherals that might be connected to the system. Remove the housing. Connect the AC power cord. Warning: To prevent injury, avoid contact with the bus bars and the bus bar screws (shown below) when the computer is plugged in and powered on. The bus bars and the bus bar screws, located at the top of the main boards, have enough energy to cause a burn if they are bridged with metal (such as a ring or other jewelry). Avoid contact with I/O board bus bars and bus bar screws: 
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. Important: Be sure to remove any magnetic source used in step 6 (above) to re-enable the safety interlock circuit as soon as possible, to minimize risk of injury from contact with the bus bars and bus bar screws.