Thinkpad L540, no post until I switch ram, this one is weird

[Insomniac1971]

This is a very weird problem and super annoying, so if someone who has experience with this, can explain why this is happening, then I will be grateful

This happen usually when the laptop has been OFF charger for some hours, but has also happened when laptop has been shut down and left ON charger over night :

I turn on the laptop normally by pressing the power button briefly, then power comes on and Power LED and Fn LED turn ON/OFF constantly (no post and no beep codes)

If I(and this is my only option at this point), press and hold the power button then a long beep until I let go.

This started about 3 months ago,

Here come the strange part, if I switch the ram modules (2x8Gb correct model, and checked with memtest86+ for 4 hours to be 100% ok), and start the laptop then it will post(though a few seconds more than usual) and boots into Linux Mint 21.3 Cinnamon(since this happens before boot, I doubt its OS related, but you never know).

It doesn't matter which module is in the laptop and which position, because I already tried with the original ram, and same thing happened - took the ram out and put it back in and the laptop posted and booted.

So I doubt its Ram related, and the delay in post, I believe is because of changed configuration(both power cable and battery were disconnected, but RTC battery were attached).

I have changed the RTC battery to one I bought in 2023 and know is working, and laptop posted and booted - until one day it didn't.

There is no patern, it does this randomly.

As I said laptop worked fine until about 3 months ago, so doubt it has anything to do with my upgrades since most were done 1 year ago :

New keyboard(Original is JP, new is US) 

New LCD original AUO 15.6"HD

Upgraded to 2x8Gb ram

Replaced the 500GB HDD with Crucial BX500 500Gb SSD

Bought new 100Wh aftermarket battery(but also happens if the laptop has been OFF charger, but used original battery which still have 43Wh out of 56Wh).

Bios version is newest, that was the first thing I did when I got the laptop in 2024.

I can get the laptop to post by removing a ram module, but next time it will not post, I add the one I removed and it posts.

As I said super weird.

I am an retired IT Professionel and technician, but I don't do board level repair. 

Its not a fan or thermal paste problem, I already checked this and replaced the thermal paste with a good one(not liquid metal), and fan run for a few seconds when I press power button, and when the laptop do ON/OFF. 

I tested without SSD and with SSD and it makes no difference. 

If it were constant and only OFF charger, I would say motherboard or RTC(though I replaced it and there is no failure), but its not. 

And as soon as I make any change to the hardware, it post, which lead me to think it might be a bug in the bios, that just surface now. 

What do you think? 

 I thank you in advance for your help

Jan

Model 20aua0f4jp 

[RealBlackStuff]

Does it have a locked BIOS?
If so, read this, specifically posts #7 and #9.

[Insomniac1971]

Hi RealBlackStuff and thank you for the reply, but its not a locked bios problem I can change whatever I want in bios (as there are no bios password), and it is saved when I exit.

Also, I believe we can rule out the RTC battery, because there is no error on this and its keeping the settings, clock/date.

[RealBlackStuff]

Have you tried a BIOS update?