Lenovo ThinkBook G2 ARE- how to diagnose on/off

[someonenotyou]

Hi.

I have a laptop Lenovo ThinkBook G2 ARE, which is switching on and after 3 seconds off (Esc, Caps Lock and Num Lock key lights on)?

After leaving it to sit for few days, it would work for 1 hour, then it would switch off.
When switched on again, it would get to Windows boot screen and switch off.
After 3 times like this, it would just light up Esc, Caps and Num lights on keyboard and switch off after 3 seconds without anything on screen.

I have tried a different BIOS, disable on-board RAM with the script, and reflow the RAM chips. Nothing helps.

Where should I look, if the laptop is half-alive, with no shorts?

Maybe the on-board RAM is defective and the disable script does not work on this laptop. What do I need to desolder to disable on-board RAM on hardware level?

Thank you.

[someonenotyou]

It seems, that I found the problem - water damage on external board, where power button is. There were some corrosion on pins going to power button.
Maybe with temperature change it short the power button and switched te laptop off.