MacBook overheating despite changing thermal paste.

I have Macs fan control installed on my MacBook which was given to me around 2015. I used to use this machine to play games, such as Cities Skylines, but nowadays, I only use this machine for writing, emails, and other daily simple tasks such as internet browsing.

This used to overheat years ago but that was because its fan/vents were clogged. (It was never opened since bought) At the time, when under heavy use, it usually hovered around 80-85C.

Fast forward to this year’s January, its battery (a replacement one) started to cause some issues like skyrocketing temps and slow/laggy responses. So, I decided to not use it while I was waiting for its second battery replacement. When it finally came and was swapped (and recalibrated), I noticed that its temp often shoots somewhere around 100C. It was really strange because I know this machine only reaches 85C max upon boot up, even under heavy load. Now, knowing its age, I assumed its thermal paste has reached its stop. So, I properly changed its thermal paste and cleaned its fan again.

At first, I used a cheap paste, specifically Deepcool z3. The temps skyrocketed, now reaching the CPU’s “junction temperature” which is 105C. Not only that, but it started showing obvious signs of overheating such as sudden shutdowns. At first, I thought I did the application wrong, so I redid it. Three times. Same result.

After having enough, I decided to buy another thermal paste, Arctic MX5. Sadly, the temps continued to touch the 105 mark and sudden shutdowns persisted, along with other “glitches” such as random log outs and whatnot.

After doing some quick research, I tried running “pmset g- thermlog” on terminal while the machine was at 105C. it was really throttling, the CPU speed was dipping 100. I tried resetting SMP and PRAM/NVRAM but to no avail. I redid the application and the results were the same. I did wipe everything, thinking there might be some problem with the software, and did a fresh install with its original installer CD then update its software from there. I also tried running AHT while on its original software, it found nothing.

Back to its maximum macOS version, I find something very odd. I tried running the “yes” command and the temperature didn't touch 95C. I ran the “thermlog” again and this time its cpu speed remained 100 despite reaching 105C.

After all those time consuming tasks, the problem remains. I always keep my terminal running “pmset -g thermlog” together with activity monitor and the temps on Macs fan control while doing my everyday things. Sudden shutdowns still occurs.

Now, I have heard some theories or came up with some, all of which are hardware-wise.

Could it be its second battery replacement that is causing the problem, perhaps its first replacement battery left a permanent damage?

Could the temperature sensors on the CPU diode, somehow, become faulty considering its age? Could it be its heatsink? Oversensitive sensors?

If the culprit is found and beyond my control, then I guess I just have to wait for this laptop to finally retire and buy a replacement.

EDIT

The cpu temp is basically spiking under a few seconds (and when launching apps or loading websites via safari), unlike previously where it gradually rises up for half a minute or so.