Voltage rails fail after 4 sec

[senatorius]

Hi,
trying to fix a ThinkPad T420 for, fun and to reduce e-waste. I'm comming up on a peculiar issue where the main 5V/3V3 rails are established when I power the laptop with the charger, but fall down to 0V after 4-5 sec. I've managed to glitch it so that it will hold the main voltage rails for longer, but then the PCH starts heating up. I wasn't able to find any info on the internet about how to fix this issue. So far was not able to find any shorted caps or something similar. Any ideas would be appreciated.r