If you got it already dead.then almost anything could have happened to it prior to you getting it. In the other thread it was also mentioned that you got this 2017 iMac from someone else (already dead).I was assuming it was your iMac. The repair costs are way higher (around 600).

If i5 7400 CPU's are cheap.you could test the CPU replacement theory by replacing the CPU. A Logic board failure (computer doesnt start, black screen) is a very common failure of (old) macbook pros. As you know.logic boards are very complex.can't really say if the logic board issue is solely due to a problem CPU (can only test this by replacing the CPU by itself). None of the diagnostic LED definitions specifically address working/non-working CPU.mostly only if the logic board is communicating properly with other components of the iMac. The computer now turns off when it wants to and even if it’s on, it is so slow it’s impossible to use. Not sure if you have the proper diagnostic LED definitions for a 2017 iMac.below I attached a screenshot of the diagnostic LED definitions for a 2019 iMac (they may be the same for a 2017). Question: Q: Failed logic board on MacBook Air 2019 I bought my laptop about 2 years ago and already the logic board and battery have failed. Click to expand.I read your other thread where you mentioned none of the logic board diagnostic LEDs are lit.

Signs of mac logic board failure