elitebook 840 G3 LED's blink 4 times, no beep, no boot after changing motherboard

[slimzs]

hi everyone

i have an elitebook 840 g3.

it all started one night after usage, i shut it down as per normal and the next day it refuses to boot. when I plugged in, the LED's did not light up, and no signs of life. i assumed that the board is dead and bought a new one.

today i installed the replacement board. i connected everything - display, ram, fingerprint, touchpad and keyboard, wifi and LTE card, everything is connected except the battery. i plug in the AC power. the battery LED is glowing orange. when i pressed the power button, the LED on the power button and caps lock glows for 4 seconds then turns off. the numlock LED does not glow.

at the side of the laptop, when i press the power button, the "power" LED glows white for 4 seconds then turns off. the "battery" LED blinks orange and white 4 times, then goes back to glowing orange as described above.

no beeps. no images on the screen.

i tried the hard reset (disconnect all power sources and pressing the power button 20 seconds), tried windows+B and holding power button for 5 seconds (in case the BIOS is faulty) but to no avail. tried booting with 1 RAM chip in each slot as well. tried powering when on ac adaptor only, on battery only, and on both AC and battery, and the above behaviour does not change.

any suggestions welcome

see attached video to see what i'm talking about.

[slimzs]

update:

read somewhere that blinking for 4 times usually indicates a hardware fault

after much troubleshooting, i managed to isolate the cause to a damaged ribbon to the keyboard (touchstyk)

after disconnecting, the computer boots up normally

would have to make do without the touchstyk now, but thankfully no need to junk the whole computer

keeping a record here hopefully it would be useful to another person.