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.
However, I can describe in such a schematic if you are troubleshooting or repairing a board with that code: la-g851p schematic
Use the schematic to find the VCC pin on the BIOS chip (usually Pin 8). If there is no voltage here, the board will never boot. The LA-G851P is notorious for EC firmware desync
You can trace the (19.5V) from the DC jack through the first and second MOSFETs to the main current-sensing resistor. This is where most "no power" issues are solved. 4. Signal Communication la-g851p schematic