Assuming you have the LA-G851P open to page 35 (Power Sequencing), here is the linear path for a cold boot:
If stuck at step 4: The EC is dead or the RTC circuit (battery/CR2032) is corrupted. Check the schematic for RTCRST#. la-g851p schematic
Without a schematic, motherboard repair is essentially guesswork. The LA-G851P diagram provides the following critical information: Assuming you have the LA-G851P open to page
LG Grams use a barrel jack, but the charging logic is complex. If stuck at step 4: The EC is
The LA-G851P is notorious for EC firmware desync. The schematic shows the EC is connected to SPI ROM U49. If the board turns on but shuts off after 3 seconds, reprogram U49 with a CH341A programmer.
When a laptop like this arrives on your bench dead or with a specific fault, the schematic (often accompanied by a board view file) is essential. The LA-G851P schematic provides: