Fails to post, tried replacing motherboard, can't diagnose the issue

I have had this Lenovo Yoga 3 Pro laptop for 5 or so years but I haven’t used it in over three years. I decided to pull it out of the closet and attempt to revive it and it has lead me down a long and arduous road.

Symptoms:

1. Battery charging appears to work. Charging it with the adapter exhibits the correct LED behavior over time for empty, charging, and full.
2. Pressing the power button lights up the power LED and lights up the keyboard. After about 3 seconds, the keyboard LED turns off but the CPU fan keeps spinning. Nothing appears on the screen.
3. Identical behavior for #2 if I press the special BIOS button next tot he power button.
4. The screen appears to be working correctly (see: “After installing the new motherboard“ below)

What I tried:

1. I tested the battery with my basic understanding with a multimeter and got a proper voltage out of it. All above symptoms are the same with just battery and no AC connected. Behavior is the same if I also disconnect the battery and only connect to AC.
2. I tested the motherboard with a multimeter. I don’t know very much about electronics, but from very rough research it looks like all the components are getting their proper voltage, I checked the fan, keyboard, and usb ports.
3. I purchased a replacement motherboard off ebay. This is where things got weird:

After installing the new motherboard:

1. The screen came on and showed the Lenovo logo with the test “Checking new devices” and then proceeded to shut back off and exhibit the same original symptoms.
2. One time while experimenting, it showed lenovo boot up screen followed by a windows BSOD. I was never able to get it to do anything else after that.
3. I thought since I couldn;t get to the BIOS that I could remove the harddrive and leave in a bootable SD card for an unbuntu install connected to force it to have boot priority. This did not work… but after leaving the laptop plugged in over night it was attempting to install ubuntu when I woke up the next morning! It was stuck at 38% of checking the SD-card content and never continued. I havent been able to get it to do that again.

What should I check next? Surely even if everything was disconnected except for the keyboard and monitor the device SHOULD boot into BIOS right?? Suggestions and guidance welcome, this is my first time attempting to repair electronics at this level.

New Developments:

SO I have figured out that when the laptop is completely off-power (ac and battery both disconnected) for at least a minute (maybe 30 seconds?) it resets something internally that allows it to try booting again.

From this I am able to try a whole new host of things for more data:

1. Booting into windows actually works. But either at the login screen or within seconds of logging in it BSODS.
2. I am able to access the bios
3. Automatic recovery from the Novo key fails with a BSOD
4. With the ability to boot from USB I have tried running ubuntu: it boots up but then freezes at some point during the launch and becomes unrespsonsive
5. I also tried booting memtest86, which gets past the introductory scan, counts down to start the test, and then black screen unresponsive.
6. Booting into windows SAFE mode ACTUALLY SEEMS TO WORK, or at elast doesnt bsod immediately…. my main problem is i dont rememebr the password for that account so i cant actually login (safe-mode disables the PIN login)
7. I also took out the harddrive and loaded it into my desktop PC and scanned it and got full SMART green across the board.
8. I’ve done all of the above both on AC power only and on only battery power. Which to me rules out issues with either.

At this point it seems like the motherboard (and probably the old motherboard as well) as well as the hard drive and power are all fine. Could it be faulty RAM at this point?

I finally got memtest86+ to run

With Memtest86 failing (logs show it entering a graphical mode before it quits, I was able to get the old memtest86+ to run by disabling UEFI in the bios and yanking the harddrive. I very quickly started gettign errors. Up to 2million errors ocurred before i stopped the test:

Now the RAM is soldered onto the motherboard, and this is a new motherboard, so I’m definitely at a loss now. Temperature-based???

Update

So the next test was keeping the cooler cold:

When I did this, it booted into windows and stayed stable and running for several minutes. When I removed the fan and ice, the system crashed within a minute.

Heat issues would explain the random BSODs from missed memory timings and hardware failures that always come after a somewhat set period of time. The time to crash also shortens the longer I sit there continuously trying it.

The CPU fan slowly weakening and burning out would also explain the slow decline I experienced with the laptop when I initially started having problems, which was increasingly common bsods. This ultimately likely resulted in the previous motherboard dying.

We are rapidly approaching the ‘could have just bought a replacement off ebay’ price point but I ordered a replacement cooler unit and we’ll see if that fixes everything next week!