5 years ago, I bought a second hand MacBook Pro which was 18 months old, however, it wasn’t without its problems.
If you stressed the graphics card (think playing games), the computer would crash.
I was told the only way to solve this issue was to force the fans to run at full speed!
I could not accept that a merely 18-month-old computer should have a hardware fault.
By now you should know me, I wouldn’t settle for substandard hardware.
Few people know that you have the “Sale of Goods Act” which will help you get free repairs for up to 6 years,
assuming the product could be expected to last for that long (and you haven’t been misusing it).
So, I kept using the computer as Apple intended (i.e. letting OS X manage the fan speed), and the more I did, the more often the screen would get corrupted or crash.
After a while, the computer was having a problem regularly, so the next step, as a test, was to format the computer and reinstall the operating system.
This is a good test, as it helps distinguish between a software fault and a hardware fault.
The computer still had problems. So, I booked it in to be looked at by Apple.
As luck would have it, I found that Apple had extended the warranty for one specific issue:
Showing that page to the Apple Genius, they agreed that I was covered, and they would do the repair for free.
(It is always easier to use warranty than having to make a company repair under the above mentioned “Sale of Goods Act”).
Unfortunately, the repaired machine I got back was far more broken(!), and Apple tried to fix it twice more(!),
before giving up and replacing the laptop with a brand new model.