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When I attempt to turn on my newly built custom computer, it powers on for 5 seconds then turns off again. The only happens when I have the ATX CPU power cable connected. When this cable is removed and the CPU isn’t powered, the motherboard powers on but doesn’t do anything.

  • CPU: Intel Corp i5 7400
  • GCard (the same with or without the GPU): Asus NVIDIA GTX 1050 Ti
  • RAM: 16 GB (2x8)
  • Motherboard: Gigabyte Z390M Gaming

This is not overheating, as the CPU doesn’t even seem to turn on!

  • 7
    You're going to need to start troubleshooting this from the ground up. Remove any non-critical component (all but one stick of RAM, HDD, GPU) and see if it will boot. I would suggested tearing it all apart and working from scratch again, double checking all fits and connections. – Michael Frank Dec 30 '18 at 21:10
  • The same, it doesn’t matter what I connect to it. If I connect Power Supply to the CPU power on the motherboard, it will not work. Maybe this is because of some compatibility issues between motherboard and CPU? – Andrey Tsurkan Dec 30 '18 at 21:13
  • Can you confirm your motherboard model? I can't find that model listed anywhere. – Michael Frank Dec 30 '18 at 21:17
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    Then yes, the motherboard and CPU are incompatible. You have a Kaby Lake CPU and a Coffee Lake motherboard, while they are both LGA1151, they are different enough that they will not function together. You should take care to view the Supported CPU list when buying a motherboard: gigabyte.com/Motherboard/Z390-M-GAMING-rev-10#support-cpu – Michael Frank Dec 30 '18 at 21:23
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    9th generation hardware is backwards compatible with 8th generation hardware. 7th generation isn’t compatible with anything except 7th generation hardware. – Ramhound Dec 31 '18 at 1:40
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Your CPU is not compatible with your motherboard. It is from a previous LGA1151 generation and your new motherboard chipset does not support this CPU.

Your Intel i5-7400 processor is a Kaby Lake processor which is not compatible with the 300 series chipset on your motherboard. You will need an LGA1151 Revision 2 CPU, something from the Coffee Lake range. An equivalent CPU would be either an i3-8100 or an i5-8400. Either of these would be an improvement over what you currently have (and would actually work).

See this chart from the Wikipedia Article on LGA1151 processors:

Coffee Lake processors support

https://en.wikipedia.org/wiki/LGA_1151

You can also verify this by checking the CPU Support page listed on your motherboard manufacturers website:

https://www.gigabyte.com/Motherboard/Z390-M-GAMING-rev-10#support-cpu
(Unfortunately the table is too large to screenshot, however the minimum currently supported i series CPU is the i3-8100t)

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    Even with a compatible socket, and the CPU fits, it's still a no-go. Disappointing that cpu's don't follow the cat rule "if it fits, I sits." Interesting, +1 – Xen2050 Dec 30 '18 at 22:38
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    The reason this is happening is that the PCH (Platform Controller Hub) on the motherboard is very tightly coupled to the CPU microarchitecture nowadays. Adjusting the physical characteristics of the CPU every year is unnecessary and would add cost for Intel. So we're left with physically and electrically compatible CPUs that don't work because the PCH is too old. Newer PCHs can generally handle older CPUs, but not always. – allquixotic Dec 30 '18 at 22:41
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    Sometimes they are specifically designed to support multiple generations depending on the tick-tock-tock cycle. – Ramhound Dec 31 '18 at 1:43
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    Intel's followed a strict 2 year compatibility block policy with CPU/chipset's for the last decade or so. While they haven't spoken publicly about why the still did a 2y break when 10nm was delayed and the released a 2rd LGA1151 generation; but avoiding a repeat of the "will it work" chaos from when they used LGA775 for ~8 years worth of CPUs which is currently being repeated with AMD's AM4 socket where many mobo makers are dropping support for older CPUs to avoid needing to use a larger flash chip to store configuration/startup data is likely. – Dan is Fiddling by Firelight Dec 31 '18 at 13:41
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    It's probably better to just reuse the same socket but just refuse to power on, vs using a siiimilar socket (LGA115X) which kinda-sorta fits then when you clamp it down you destroy the socket and/or processor. – Nick T Dec 31 '18 at 21:01

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