2-1 LED code indicate a system board ( motherboard) failure. 

But in case your case it seems like a Corrupted firmware due to 2 main reasons.

1 Error messages (ME FW Downgrade – Request MESpiLock failed)

2 You are able to get in to BIOS and ePSA daignostics

I would strictly recommend to get in to OS with whatever method works for you and take a backup.  

Only applicable solution prior swapping the board would be to Recover BIOS 

https://www.dell.com/support/kbdoc/en-in/000132453/how-to-recover-the-bios-on-a-dell-computer-or-tabletvv

I would recommend the second USB method as it will wipe the current BIOS from Machine and use the latest Downloaded firmware on USB drive. 

Thanks for the quick response XPS_Man.

I eliminated a bad motherboard since the 8930 will boot when AC cord is disconnected and operate normally. The three Dell hardware tests resulted in no hardware issues found. Then why does the Dell boot process show a motherboard failure when the Dell hardware tests say otherwise? I have read that Dell boot diagnostics can be inaccurate. And sometimes indicate the last good step in the boot process.

I am backing up every night using Acronis True Image 2021. The backups are passing validation. I have created an Acronis Universal Restore USB. So I am good with backing up and restoring data files and system files.

I have completed a BIOS Restore but did not include it in the above details. On the fifth attempt the 8930 booted into BIOS Recovery. I inserted a USB containing "BIOS_IMG.rcv" on an USB drive. Then followed the instructions in the link you provided. The BIOS version was 1.1.16 before the BIOS Restore and 1.1.17 after the BIOS Restore.

The next step seems to be replacing the motherboard. I wanted to exhaust other solutions before purchasing a new $230 piece of hardware.

You are following correct troubleshooting step.

One more suggestion, 

If the machine is less than 4 years old. 
Spend that money to get annual warranty instead. 

Thanks XPS_Man

@Alan54Neal  Get to the desktop however you can get there. Then disable Fast Startup inside Win 10 (That's not the same as Fast Start in BIOS setup):

1. At the desktop, open the Windows Power & Sleep screen
2. Click Additional power settings
3. Click Change what power buttons do
4. Click Change settings that are currently unavailable
5. UNcheck Turn on fast startup
6. Click Save changes
7. Do a full power-off shutdown, not just a restart
8. Power on and boot...

NOTES:
When you boot at step #8, be patient because Windows may install some updates which can only be installed when Fast Startup is disabled, so this boot may take longer than "usual".

Assuming that solves the problem, you can re-enable Fast Startup at step #5, above, because the PC will boot noticeably faster when Fast Startup is enabled.

Let us know if this helps...

Thanks for the response. I did uncheck fast start but did not include it in the details above. This change did not fix my issue.

My 8930 may have a hardware or software problem that is preventing a restart. Some steps I did require a restart to complete. Can not having a restart after completing these steps result in corrupted upgrades? The Dell quick test and full test did not show any hardware problems. Can Dell hardware test miss finding a problem?

Thanks for your time.

@Alan54Neal  - Fast Start (in BIOS) and Fast Startup (in Win 10) are two different things, so which did you disable?

Before going further, run the PSU BIST test.

BTW: Is the monitor connected to the add-in video card, not to any of the onboard Intel video ports?

Open BIOS setup and see if SATA Operation is set to RAID or AHCI, but don't change it. Also verify that Windows Boot Manager is set as first boot option in BIOS.

Have you tried re-seating everything on the motherboard?

1. Power off, unplug
2. Press/hold power button for ~15 sec
3. Open case and remove motherboard battery
4. Press/hold power button for ~30 sec
5. Reseat RAM modules, PCI-e cards and all cable connections to/from the motherboard
6. Reinstall battery
7. Close up and connect mouse, monitor and keyboard
8. See if it boots without errors

If it gives the errors, remove the add-in video card at step #5, and connect monitor to an onboard Intel video port at step #7.

If you get the errors again, now disconnect the add-in HDD from motherboard and remove all RAM modules at step #5. When you boot up, listen for and count beeps from the tower and see what amber blink code you get now.  The presence/absence of beeps and the blink code this time may give you a better clue about the cause of this failure, but here are some other things to consider after seeing what happens without any RAM.

Reinstall one RAM module in the 2nd slot from the CPU and reconnect the HDD at step 5.  (You can leave the add-in video card out for now.) See what happens when you boot.

Create a bootable Win 10 USB stick at the Microsoft site. You can do this on any PC. You need at least an 8 GB stick, but no larger than 32 GB. Plug that bootable stick into the XPS 8930 with power off. Power on and tap F12 to open the boot menu. Select the option to boot from USB.

When the installer opens click: Repair your computer > Troubleshoot > Advanced Options > Troubleshoot > Startup Repair.  Let that run and see what happens when you boot now...

I disabled the fast startup in WIN 10 using the instructions you provided above. The result was no boot and 2 + 1 amber lights on the power button.

The PSU BIST test resulted in LED light did not light up and PSU fan did not run. I ran the test three times with the same result. So the PSU failed all three BIST tests.

I also ran the "How to Run a Power Supply Unit Self Test on a Dell Desktop or All-in-One Computer" at this link https://www.dell.com/support/kbdoc/en-us/000125179/how-to-run-a-power-supply-unit-self-test-on-a-dell-desktop-or-all-in-one-computer  and "Steps to Confirm that Power-Supply Unit is Defective" test at this link https://www.dell.com/support/manuals/en-us/xps-8930-desktop/xps-8930-service-manual/steps-to-confirm-that-power-supply-unit-is-defective?guid=guid-60739a4b-845e-4f20-85bb-ad1bce8a0b08&lang=en-us. This BIST test used the BIST button and not the AC cord. The results for both tests are LED lit but only for as long as the BIST button is pressed. Instructions are not clear about how long to hold the BIST button. A quick press results in LED not lighting. And the PSU fan did not spin in all attempts. I tried the test three times with the same results.

The monitor is directly connected to a NVIDIA GeoForce GTX 1050ti GPU.

The instructions contained in "Have you tried re-seating everything on the motherboard?" were completed. The 8930 tower was built in 2018. The towers built from 2015 to 2020 are the first to not have beep codes. All this 8930 has is blink codes. The 8930 booted to a white light on the power button. Then the 8930 restarted and failed to boot. Blink code is 2 + 1 amber lights.

Since the above test gave an error, I removed the add-in video card at step #5, and connected the monitor to an onboard Intel video port at step #7. The 8930 started to a white light on the power button then restarted. The restart failed with 2 + 1 amber lights.

Since the above test gave an error, I disconnect the data HDD and the backup HDD from motherboard and removed all RAM modules at step #5. The start failed with 2 amber lights, not 2 + 1 amber lights. The 8930 did not restart.  I also removed the SDD in addition to the two HDDs in another test and the 8930 started with 2 amber lights, not 2 + 1 amber lights. The 8930 did not restart.

I reinstalled one RAM module in the second slot from the CPU and reconnected the two HDDs and SSD at step #5.  I left the add-in video card out. The boot resulted in a power up with a white light on the power button. The 8930 then restarted to 2 +1 amber lights.

I reinstalled the other RAM stick and GPU. I created a bootable Win 10 USB and selected F12.  I selected Boot Options and the 8930 booted to the sign in screen. It will always do this. My go around is to select F2 and change BIOS Setup to Secure Boot disabled and Attempt Legacy Boot Enabled, then select F12. Then the option to boot from USB is available. I selected USB Device. WIN 10 started and I selected Startup Repair. The screen changed to “Attempting Repairs” then restarted and failed with 2 + 1 amber lights. I performed a forced shutdown, unplugged AC cord for 15 seconds and started 8930 again. Start failed with 2 + 1 amber lights for a second time. Repeated these steps again and the 8930 booted to sign in screen. The “Repair” process is failing maybe because of the failed restart and failed starts.

Please let me know what do you think of these results?

Sounds like the PSU may have failed...

Lots of threads about replacing the PSU in the XPS 8930 so search for them.

I appreciate your help in diagnosing my 8930. I've marked your last response as the solution to my issue.

@Alan54Neal  - You're welcome.

Obviously, I can't guarantee a new PSU will solve the problem, but that's a lot cheaper than buying a motherboard.

Post back and let us know what happens...

I ordered the new PSU yesterday. Should be here by 09/13, so it will be awhile for an update.

The start and restart failures may by caused by a hardware problem. As you commented the PSU was least expensive place to start. And the no post may be caused by a corrupted BIOS or IME driver from a failed update due to a failed restart.

Thanks for help RoHe, will post the results.

Installed the new PSU. The new PSU is same Dell model number, D460AM-03. The same issues are still present: two error messages (“(A7) ME FW Downgrade – Request MESpiLock failed” and “Error sending end of POST message to ME:HECI disabled, proceeding with boot”), failed restart and failed start. Starting the diagnosis of the motherboard. I can not find a helpful explanation of what “(A7) ME FW Downgrade – Request MESpiLock failed” means. This information would start me in the right direction.

Thanks for your time RoHe.

Sorry to hear the new PSU didn't help. Try this:

1. Download BIOS Recovery Image File, BIOS_IMG.rcv, onto an empty (non-bootable) USB stick that's formatted FAT32. You can do this on any Windows PC.
2. Connect a wired USB keyboard to the XPS 8930, borrow one if you don't have one
3. Plug the USB into the XPS 8930 with power completely off
4. Power on and immediately press/hold CTRL + ESC keys until the BIOS Recovery page appears
5. On the BIOS Recovery screen, select Reset NVRAM (if available) and press Enter
6. Select Enabled (if not already selected) and press Enter
   
7. Select Recover BIOS and press Enter to start the recovery process

NOTE: PC may reboot itself several times during the process. Just leave it alone. Don't power off or press any keys etc, even if it appears to have shut itself down completely, until it's -hopefully- back at your desktop,