Dell 7573 won't POST

Hello,

My Dell Inspiron 15 (7573) will not POST. I'm having trouble diagnosing the problem.

Recently, I inherited an Inspiron 15 7000 2-in-1 (7573), and I've discovered that it won't POST or start up correctly. My symptoms are as-follows:

When I press the Power button on the laptop, the white indicator LED light turns on and then briefly turns off. The the keyboard immediately lights with the Caps Lock button on. The fans start on full. This continues for ~5 seconds and then the white indicator LED starts blinking. The laptop then restarts with the same LED, Keyboard, fan behavior. Afterwards the white indicator LED starts blinking every ~2 seconds indefinitely.  In all cases the display remains blank

I have attempted to boot after each of the following:

• Cycled Flea power (per service manual); attempted several times for durations up to ~5 min
  
  • Did it for at least ~2 mins of each configuration: without CMOS battery, with main battery; without main battery with CMOS battery; with both
• Attempted to reset the CMOS settings by removing the CMOS battery and main battery; up to 15 minutes idle without CMOS
• Installing a new CMOS battery
• Installing a new I/O board (with changing the CMOS batteries)
• Removed SSD
• Removed RAM (one 16GB DIMM); changed DIMM slots
  • Removing the RAM does produce the Diagnostic light pattern of 2 amber flashes/3 white flashes
• Removed main battery
• Removed each combination of two (SSD, RAM, Battery)
• Used two different 65W adapters
• Performed an LCD display BIST
  • The display will enter in the BIST and show  the White/Black/Red/Green/Blue screens. It does not go to a follow-up screen
• Attempting to boot after holding the Fn key before pressing the Power button

Does anyone have any insights before I consider having this laptop serviced?