HP Elitebook 2540P Not power on

Model : HP Elitebook 2540P
Board id : Compal LA-5251P

Laptop not power on. When i press power key, laptop power led blink a few seconds and then off. No CPU Fan spin. +5ALW and +3ALW are ok (When i press power key ). But +5VS and +3VS are not found. There is no RUN ON Volt. BIOS flashing is not correct the problem. How can i find the problem device. Please help me.

Measure the voltages on all pins of PU502.

PU502 ( Without press power key ) - All pins are 0V

PU502 ( Press power key )
Pin 4 and Pin 10 - 5 V
Pin 14 - 4.75 V

Other pins are 0V

Measure the voltages on all pins of Q38, Q50, Q42, Q44.

Q38 ( Press power key )
Pin (1) Drain - 0V
Pin (3) Source - 3.4V
Pin (2) Gate - 3.3V

Q50 ( Without Press power key )
Pin 1 and 4 - Ground
Pin 2 - 0V
Pin 3 - 3V
Pin 5 - 0V
Pin 6 - 0V

Q50 ( Press power key )
Pin 1 and 4 - Ground
Pin 2 - 0V
Pin 3 - 3V
Pin 5 - 0V
Pin 6 - 3.4V

Q42
All pins - 0V

Q44 ( With or Without Press power key )
Pin 1 and 4 - Ground
Pin 2 - 3.1V
Pin 3 - 0V
Pin 5 - 3.1V
Pin 6 - 0V

Measure the voltages on pins 34, 41, 74, 75, 81, 85, 93 of KBC
Measure the voltages on pins 4, 79 of the KBC in the following cases:
at power off
with pressed power on
to power on.

KBC Pins Volt ( Without press power key )
Pin 34 - 0
Pin 41 - 3.1
Pin 74 - 3.1
Pin 75 - 3.1
Pin 81 - 0
Pin 85 - 0
Pin 93 - 0
Pin 4 - 0
Pin 79 - 3.1

KBC Pins Volt ( Press power key )
Pin 34 - 0
Pin 41 - 3.1
Pin 74 - 3.1
Pin 75 - 3.1
Pin 81 - 0
Pin 85 - 3.1
Pin 93 - 0
Pin 4 - 3.0
Pin 79 - 3.1

Measure the voltages on all pins of Q43.
Lack of communication with the PCH.
Reballing / better replacement.

Q43 ( With or without press power key )
Pin 3 and 6 - 3.1V
Other pins - 0V

Yes. I think, you're right. I reflow PCH with hot air gun and laptop is start OK. But only once. When i shut down and reopen laptop, it does not start again. I don't have BGA Rework Station (or) BGA Reball Stencil. So I can't test reballing and resoldering.

To fix it, you need to replace the PCH or buy a working replacement board.

Yes. whatever it takes, thanks for your helping and teaching. Thanks a lot.