HP Pavilion Notebook Screen Randomly Dims

[MaxProAndU]

We're experiencing a "phenomena" with our last-standing HP Pavilion G7, a g7-1365dx / A7A33UA#ABA. It has a replacement board from a donor g7-1330dx by the looks of the BIOS. Both G7's are one of the many Pavilion G series laptops that support a HP 649948-001 motherboard, and that's the part number on the label.

Here's what happens. Regardless of which OS it runs (Microsoft Windows 10 Home / Linux Mint MATE 20.04), the brightness icon will appear and the screen starts to dim until it reaches it lowest setting. The icon stays on the screen, just as if you're holding down the F2 button on the keyboard (F2/Dims F3/Brightens).

At this point it becomes a "video game" adjusting the brightness back up. If you hold down the F3 button it'll usually let you return the screen to max brightness. But as-soon-as release F3 the laptop continues to dim as if F2 is being pressed. Sometimes, if you just hold down the F3 key long enough, the laptop gives up and just stays at max brightness.

Then there's the "challenge/boss battle"! This is when it continues to dim even as you hold down the F3 key. But if you start tapping the F3, the level tries to go up. The faster you tap, the quicker you push the brightness level to the top. Once you reach the top and stop tapping, only trying to hold down the F3 button, it can easily start to dim again as if the F2 is being pushed. This has you tapping away like mad, once again until the laptop gives up and lets go of the F2 key.

We're not sure where the "ghost in the machine" is. We've found nothing online and our local HP guys giggle and just don't care. Any ideas?

[MaxProAndU · Accepted solution]

Turns out the problem was related to the fan & heat sink.

Dirt was blocking the cooling fins and the thermal paste was no longer effective. The heat generated by the processor was directly below the area of the F2 key, some how enabling a contact. Mind you, this accrued using two different keyboards. Once the fan & heat sink were serviced, the machine ran much cooler and the problem disappeared.