GPU (RSX) bad joint (even shortcut - generating heat > internal thermal sensor > syscon >> the fan spins like a jet plane). No IHS removal unless you know what you're doing!!!!
Applying famous Arctic Silver is a BIG mistake, better use Arctic MX2, MX3, MX4 or even that "white" silicone thermal paste (cheaper). Overheated consoles ends to this. You may try reball the GPU (RSX) but is expensive. Reflow the GPU (RSX) is cheaper but you will never know what will happened when you power on your console (a big spark or not even spinning that loud fan?).
Repairing old CECHx is a waste of time, you will never know when it will fail again (a day, 10 days, a month...)