All 8 red lights on the memory boards and no post

Diagnostic LEDs

The Mac Pro logic board includes a set of LEDs to help service providers troubleshoot the computer. The LEDs are located toward the rear of the logic board, under the memory cage, next to PCI card slot 1. You can view these LEDs by removing the computer’s side access panel and looking through the memory cage to the logic board below. LEDs 2, 3, 4, and 5 are normally off and will automatically illuminate if an error occurs. To read LEDs 1, 6, 7, and 8, you must press the

DIAG_LED button, which is adjacent to the LEDs. To press the DIAG_LED button, use the nylon probe tool

@thattechman you are referring to the diagnostic LED's on the memory riser card:

Memory Riser Card Diagnostic LEDs

Both of the memory riser cards include diagnostic LEDs for each DIMM. Each of the LEDs will light if it detects an issue with the corresponding installed DIMM. These LEDs will also flash briefly when the computer is started up or shut down and when it goes in and out of sleep mode. This is normal behavior.

Troubleshooting:

Shutdown and restart the computer.

Verify EFI and SMC firmware versions and update them if necessary.

Reseat the memory riser card associated with the failure LED. If the failure LED persists, continue with the next step.

Swap upper and lower memory riser cards A and B. If the symptoms follow the riser card, continue with the next step. If the failure LED remains associated with the same memory riser card slot, consider a possible connection issue with the logic board. Replace the logic board in this case.

Reseat the appropriate DIMM corresponding to the failure LED. If the issue persists, continue with the next step.

Check memory installation instructions for proper installation order.

Swap affected DIMM with known good DIMM.

Try moving DIMM to another slot (within the same bank of two) to see if the failure LED follows the DIMM. If so, replace DIMM with known good DIMM. If the failure LED persists, replace the memory riser card associated with the failure LED.

Memory

The Mac Pro computer has two memory riser cards with a total of 8 memory slots. On each card, the slots are arranged as two banks of two slots each. The computer comes with a minimum of 2 GB of memory, installed as a pair of 1 GB FB-DIMMs. Additional pairs of 1 GB, 2 GB, or 4 GB FB-DIMMs can be installed in the open DIMM slots, as illustrated below. Maximum memory is 32 GB.

DIMMs must be installed as pairs of identical size and type, from the same vendor. In the illustration below, like-colored DIMMs must match.