Hi @Fikish ,

It's not a "Reserved" error code but a S4 state regarding "PCI device configuration activity is in progress or PCI device failure was detected",

It could be led by a failure/unsupported PCI device or a failure PCI slot. As per Dell's recommendations, 

1. Clear the NVRAM.
2. Remove all external devices and expansion cards. Replace each device one at time until the faulty device is isolated.
3. Replace the Motherboard if all expansion devices appear to work.

For point 1, try to remove PSU power cable as well CMOS battery, press and hold the power button for about 10 seconds, repeat several times and even left it over 30mins.

For point 2, remember to test each device one at a time to locate the root cause.

Also, test if the original Quadro 6000 and Tesla 2050 works in other PC.

If problem still, you may start thinking of point 3.

Just to be specific about the USB header - I didn't unplug the ribbon cable, just the power.

I just went through an ordeal that started with replacing the power supply in my T7500 and culminated in going through 2 risers, 3 motherboards, and 2 power supplies (not including the original dead PSU). On my 3rd motherboard, I couldn't get past the POST process.  The system was showing a solid green 2 diagnostic light and solid green power button. This indicates it's a Pb4 code - "Reserved" - which didn't help at all.

To make a long, gut wrenching story short - I read in one Dell forum to unplug the front USB header (which wasn't intuitive or indicated by trouble codes).  After I did that - it booted.

This is what I did:

- Pop the battery out again

- Boot with 1 stick of RAM

- No riser and

- Unplugged the front USB header

It finally booted. Then I started to add back everything and had no problems at each step.

This has been by far the most bizarre and frustrating system fix / upgrade in my 35 years working on systems. It just about made me give up - and believe me - that's pretty much impossible (I'm very stubborn, or tenacious, depending on how you look at it).

Hi @J-TX ,

I know how it feel, bro. I once deal with a no-post situation for days and finally located that a defective Logitech USB receiver lead to the problem.

I am having the same problem. Tried evrything by removing RAM, Graphics Card, USB...still no luck.

Is there any way to troubleshoot this or any tool to run at Pre-boot?

I have the same problem. My pc works fine but when i install the 2nd cpu board, it stucks on 2 light. Without 2nd cpu board, it works like a normal pc. Plz help me! What should i do now?

Also, you can try the F12 after the Dell logo displays to enter the EPSa utility. There are a series of diagnostics that you can run through. Honestly, I haven't had much luck with this approach, but it doesn't cost anything to try.

Here's an article with a series of tips for workstations that won't power on:

https://www.dell.com/support/kbdoc/en-us/000125609/resolve-no-power-no-post-no-boot-or-no-video-issues-with-your-dell-computer 

Hope one of these helps someone... I know how frustrating it is chasing your tail.

Please see my most recent reply below since I replied to someone with the same issue that you have. Good luck my friend.

Try spacing the riser card - I had a PB7 pre-boot error with no associated troubleshooting tips, growing frustration, then found this solution for creating proper spacing and alignment of the (poorly designed) riser card:

https://www.dell.com/community/Desktops-General-Read-Only/Dell-Precision-T7500-won-t-boot-up-blinking-lights/m-p/4460351 

Be super-careful removing and replacing the riser so you don't bend any of the dozens of closely set pins in the 2 motherboard connectors. Also remember to properly route, then plug the power cable back into the riser.

If this doesn't do the trick, since you can't even get to a post-boot condition where other built-in tools can be used, then I'm afraid you have to drop back to a process of elimination on installed components without making logical assumptions of "there's no way this should affect the boot process".