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.
If the exact LA-G851P file is unavailable, look for the (3100 2-in-1 version). These share many architectural similarities in the power and charging circuits. la-g851p schematic
The LA-G851P schematic has various applications across different industries: The LA-G851P is notorious for EC firmware desync
Check for +3VLP and +5VLP . These power the EC chip so the laptop can "wake up." la-g851p schematic