XPS 9550 Mother board repair?? Hardware or software??

I have XPS 9550 WITH LA-361P REV 2 / TRIPOD motherboard. System will not POST (no Dell splash). If you hit power on, power light will turn on for 4 seconds and will turn off. USB is accessed during those seconds. No function keys (F2 F12) do anything so system will not get into bios setup or diagnostics. Screen blank during this process. However, "D" key + power-on will go through graphic testing so screen is working and keyboard input is obviously working.

Suspected BIOS issue but after create USB bios disk, <ctrl><esc> power cord inserted will NOT result in bios reload. No blink codes. Removed/swapped memory and no change.

I have inspected motherboard and no obvious physical damage, component all look clean. This unit just failed to boot one day. Wondering if this might be an bios issue.

So tell me if I am thinking right, if system can recognize the "D" key + power on to execute the screen test, doesn't that mean most/all of motherboard is working at least in regard to CPU? The fact that USB drive is accessed (ie, light turns on for a second), that means something is directing it to do that?

What is going on with POST test? How do I figure out what is going on? I tried to replace bios chip from eBay with current bios but that did not seem to make a difference.

I love to solve this on my own as a software issue but I am willing to send to someone who can properly diagnose and repair. I already got "ripped off" by sending it to a company in TX who claimed to do component repair but simply said motherboard needed to be replaced.

 

Not sure this will help, but worth a try: Let the battery drain to 0% and then hold the power button for about 60 seconds (or remove the battery and hold the power button for 60 seconds).
This trick was used to reset the bios when it locked the CPU to 800MHz...

 

Yes, I tried that. I even disconnected the battery and also replaced it.

 

Can anybody tell me what is working since I can do the "D" + power on to get into display test menu. Can this confirm all power rails are working? Can it confirm CPU is working? I just like to get an idea of how to address the tracking down the problem. I there any documentation of how POST is executed and to know how confirm that it even started?

 

BIOS battery is a pain to access but there are instructions on the internet.

Alternatively, you can look for blink and beep codes to diagnose. I don't think these instructions are 100% accurate but get you started. Be careful about banging around fn codes as a few XPS units have bricked as a result (search forums).

https://www.dell.com/support/articl...diagnostic-indicators?lang=en#2014_to_Present