Verify the rails are present before pressing the power button. GPU Not Detected:

If you are troubleshooting a "No Power" or "No Display" issue, these are the standard voltages to verify on the LA-C391P: Main adapter voltage (Check entrance MOSFETs). +3VALW / +5VALW: Always-on voltages generated by the standby IC (TPS51225R). +1.35VPDDR: RAM power (DDR3L). +1.0V_ALW: PCH core power. +VCC_CORE:

A schematic for the LA-C391P is essential for tracing electrical signals and measuring voltage at various test points.