Dim Display Backlight issue

[APlusIT]

Good Day Community,

I have a HP Probook 450 G4 laptop which has got a Dim screen and basically you cannot see anything on display. It does come on boot and you can very faintly see the logos as it boots. 

Pointers you need to know are as follows and what I have tried already:

1. Connected external screen, can see display and laptop working correctly.
2. I have replaced both the monitor itself with HP replacement parts as well as the monitor cable.

3. I took the serial numbers off the original and replaced it with genuine HP Parts. (Just confirming point 2)
4. Have tried using FN and adjusting brightness.
5. HP Full reset was done as advised from their website.
6. Still after a new screen and cable same issue persists and display is barely view-able.

My thought was that if the display adapter on the MB was faulty surely a external screen would not work? Any advice on what I could try or if anyone has any advice. Obviously last unit to replace beyond what I replaced would be a motherboard but want to ask advice before considering that as it would not be cost effective to repair the unit.
Thank you in advance for any replies or help.

[APlusIT]

BUMP!
So does HP have no ideas for me at all? Quite desperate here.

Any advice would be appreciated! Thank You!