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Hessel89

macrumors 6502a
Original poster
Sep 27, 2017
594
328
Netherlands
Let's say I'd want to buy a used Mac. To make there are no hardware problems I would run the Apple Hardware Test before purchasing, but how trustworthy is this test? Would there be any hardware problems this test does not spot?
 
One thing you should be aware of that the previous owner could have spilled water over the keyboard, but the MacBook still is running fine – however, if there ever is a repair under remaining guarantee or a smaller repair which you would be willing to pay, Apple will discover the water damage after opening and not do any repair without you paying a whole logic board replacement etc. (Which in most case will add up to a "total damage" price-wise.)
 
One thing you should be aware of that the previous owner could have spilled water over the keyboard, but the MacBook still is running fine – however, if there ever is a repair under remaining guarantee or a smaller repair which you would be willing to pay, Apple will discover the water damage after opening and not do any repair without you paying a whole logic board replacement etc. (Which in most case will add up to a "total damage" price-wise.)

Makes sense, and how about iMacs? Talking about the later 4K / 5K generations. Anything to look for that wouldn't show up in the Apple Hardware Test?
 
You can never be sure, of course, that some component inside the used iMac dies one day after you bought it, but the hardware test probably is the best way to check the computer as good as possible. (Together with using the iMac at least a few minutes in normal use, of course, especially for checking the display.) A spinning hard drive is one of the more critical component if the iMac is already a few years old – you could also check its health with a separate tool like DriveDX. If it already has SMART errors, it would also show in the Apple diagnostic test, though, as far as I am aware.
 
A faulty GPU chip which has been "reheated" could also fool the Apple hardware test...a major pitfall which you could fall into.

At the end of the day, it depends on how honest the seller is!
 
A faulty GPU chip which has been "reheated" could also fool the Apple hardware test...a major pitfall which you could fall into.

At the end of the day, it depends on how honest the seller is!

Interesting. How would someone go about ''reheating'' a faulty GPU? Would there be any tell tales?
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I've also heard of screen blackouts on modern iMacs. These might not necessarily show up when testing the iMac for a couple minutes. Would there be any ways to provoke these?
 
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Interesting. How would someone go about ''reheating'' a faulty GPU? Would there be any tell tales?
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I've also heard of screen blackouts on modern iMacs. These might not necessarily show up when testing the iMac for a couple minutes. Would there be any ways to provoke these?

A GPU chip can be reheated by using a hot-air gun. With magnification you would probably see the evidence that the factory solder had been "reactivated".

As for provoking blackouts - I'm not sure how they could be provoked.
 
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People even put their MacBooks with the infamous NVDIA chips in the oven for a few minutes to "re-solder" the GPU chip to the board, making it work again for a few more weeks / months.

As to the question with the provoked blackouts – if you mean the same cause here, you could run a GPU stress test app which will "torture" the GPU to the max. If you're lucky, this could maybe uncover an already failing GPU – however, this would be far from a definite test and as a seller, I would probably tell you that – if you're that suspicious – you should go and try this on the Mac of another seller.
 
As to the question with the provoked blackouts – if you mean the same cause here, you could run a GPU stress test app which will "torture" the GPU to the max. If you're lucky, this could maybe uncover an already failing GPU – however, this would be far from a definite test and as a seller, I would probably tell you that – if you're that suspicious – you should go and try this on the Mac of another seller.

I agree. If the OP needs to do an crookedeness test on the seller first!

BTW, can you recommend a good GPU stress test app?
 
BTW, can you recommend a good GPU stress test app?

Unfortunately not. I know they exist (back in the days with the faulty NVIDIA chips, people even used them trying to break those GPUs early enough within warranty), but I never needed one of those apps myself.
 
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