I have a very interesting story to tell myself. I have an iMac from 2013 (a customized i7 2.5 GHz). Suddenly a little over the 2 year warranty, the thing simply failed. I could hear the fans, but no boot, no image on screen.
Went to the Apple Service, and the diagnostic was that the logical board failed (it's always "the logical board", which means basically, the entire computer).
I paid for a replacement. It costed me 750 euros. About an year later, the fans started to fire, and I found myself suddenly like I was flying a jet plane. Went to the net, and found an utilitarian that controlled the fans. Installed it, and the fans slowed down to the regular speed. Termal sensors were displaying everything normal.
Problem was that I was finding that the behavior of my Mac was sub-pair, and nothing comparable to what it was before failure. Since there are basically nothing that allow us to check the hardware in macOS, and since I also had Windows installed (I always make my Macs dual boot), I fired up Windows, and installed Hardware Monitor (https://www.cpuid.com/softwares/hwmonitor.html). Much to my surprise, the readings were that the CPU was running at 1.6 GHz, instead of 2.5 GHz as it should and was supposed to be.
Went back to Apple Service, and the diagnose was that there was a sensor in the "Logical Board" (that damned thing) that was malfunctioning, and reporting to the OS that the CPU was overheating, and that was causing the OS to slow down the CPU. This is in a "logical board" that was already a replacement of a failed one. In little more than four years, two faulty "logical boards".
When I asked to replace that sensor, the answer was... that I had to replace the entire Logcial Board (again). Another handful of hundreds of euros, because of a faulty piece I got from Apple in the first place.
Since it was already more than two years since the repair, Apple declined any responsibility (when it was obvious that the faulty component was their responsibility. Yes, I should have fired a complaint immediately upon the first symptoms, but it happened that fans firing at full speed is a common issue in iMacs (for example, when we replace the hard drive for a new, non-Apple HD). It didn't cross my mind that this could cause a slow down of the CPU. Anyway, the answer I got from was that "it was already a very old machine (it was, back then, not yet five years old), that was out of guarantee, and I should buy a new one". WTF???
I bought another iMac (second-hand), and gave the old one to my youngest daughter. It will work at sub-speed up to the end of its life, due to a stupid sensor that Apple doesn't even replace on its own.
I am a Mac user since the time of Mac SE30 (my first computer). This means like about 28 years of Macintosh (the SE30 was launched in 1989, and I think I got mine in 1990). However, because of Apple policy, I am becoming more and more a Windows user, and I am very happy with my (custom built) PC system.