Hello there, I'm trying to fix a DELL latitude e5430 which suddenly sotpped making POST.

When i press the power button the HDD led blinks for onw second and then all the leds remain off except the power button.

The tests that I have done are the following:
- I had dissassembled the entire PC to test hardware parts like DVD reader, HDD, wifi module, GSM modem. I also tried to remove CMOS battery and the display.

- I tried a lot of times to keep pressed the power button for around 30 seconds without any result.

- I tried to perform diagnositc boot by pressing "fn" + "power button" keys, and the battery led keeps blinking with white light (all the other leds are off).

I haven't  tried to change RAM module because i don't have one yet.

The weird thing is that the pc sometimes correctly boots and it works untill reboot. I tried to update bios firmware but a reboot is needed so I can't do that.

The power supplier and the battery are both original,

Have you any tips for me?