Tonight I started having problems, meaning that my laptop started turning off instantly. I guess it is from overheating. It was happening when I tried playing some heavy game, but now it started doing it even before the OS has loaded. I opened it (thinking that it is the thermal paste). I was right - the last time (about 6 months ago) when I changed it, I apparently missed a little end of one corner on the plate of the GPU. Even the plate, that connects to the cooling way, started getting black-ish.
So I changed the thermal paste and it was fine. Though, I couldn't hear the fan working. Putting a napkin next to the exit of the fan proved it's working, so I thought "ooh, there is such good thermal conductivity that the fan doesn't even have to turn on higher speed..." Even when I turned on a game (which was strange).
5 min. later, while I was just browsing around, the laptop shut down again. Now I am writing from an open machine, looking at the fan rotating...
I don't know if it is gonna turn off again, but the laptop is quite hot from bellow and I have no idea why is that happening. And why doesn't the fan turn on higher rate? It seems to work on only 1 frequency (as if just to show that it's working)?! The CPU is not over-heating. The GPU makes the problem here...
UPDATE: After I changed the thermal paste and turned on the machine again, it went into Windows recovery mode. I was like "okey, sure...", but it froze and I was forced to restart. After this, the fan started failing. My guess is that not finishing the process of recovery mode broke the driver or some other part of the firmware, which is connected to the fan, or smth like that. Re-installing the drivers for the video card and updating the BIOS did not help...
P.S. the laptop is HP probook 4520s