HP ProBook 455 G1 blinking power light, not turning on

[MaximX85]

I have an HP ProBook 455 G1 that won't start / boot after it has been shut down and the power cable has been unplugged and plugged back in. The laptop was working fine for well over 6 months.  All of a sudden, the light next to the power cable was blinking white.  The laptop wouldn't turn on - no lights, no display, no fan activity. The laptop is running Windows 10 Pro 64-bit, version 1511, OS Build 10586.0. BIOS is configured for Legacy mode, Secure Boot is turned off.  Those were the original BIOS settings.

The problem occurs when....

1. The laptop is shut down from Windows.

2. The power cable is pulled out.

3. The power cable is plugged back in.

4. The light next to the power jack starts blinking and the laptop will not power on.

One thing I should note is that the laptop will turn on after it's been shut down as long as the power cable has not been unplugged.  Not very useful for mobile computer that's meant to be moved back and forth between home and office.

I have tried the recommended hard reset solution already where everything is disconnected, including the battery, and the power button is held for 15 - 30 seconds.  This solution does not solve the problem I'm experiencing with this laptop.

I've been troubleshooting it for a few days and here is what I found so far:

1. Shut down laptop from Windows.

2. Pull out the power cable.

3. Disconnect the keyboard flex cable (yes, you need to have the keyboard screws removed and the keyboard loosened)

4. Plug the power cable back in.

5. Reconnect the keyboard flex cable.

6. Power on and boot the laptop.

For a mobile computing device like a laptop, it's not reasonable to have to partially disassamble the device just to turn it on.  I should also note that the keyboard and laptop is in pristine condition.  There's never been any liquid spilled on it.

Once the laptop turns on everything works as expected, in BIOS as well as Windows.  BIOS has been updated to the newest version available: F.61 Rev. A, 27 July 2015.

One thing I haven't tried yet is to replace the keyboard as I don't have one handy.

[magnumquincy]

I'm having the same issue only it's with the 650 G1. I've tried your process of removing the keyboard, connecting the power adapter, reconnecting the keyboard and then attempting to power it on to no avail.  Have you had any luck in solving your issue with the 455 G1?