Question PC turns on but no display signal, and red CPU light on motherboard ?

Aug 12, 2021
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Hello,

I woke up today to find my PC running louder than usual, and not sending any signal to the monitor. There are no lights on the mouse and keyboard either. I opened it up and saw a red light on the motherboard labeled "CPU". What I've tried so far:
  • Resetting CMOS.
  • Changing the location of the RAM stick.
  • Reseating the GPU.
  • Removing the cooler, dusting it, and putting it back in.
  • Changing thermal paste.
I just hope it's not the GPU that's the problem, as it's the only part that's not under warranty. I don't even know if that red light could somehow indicate GPU failure, given that it says CPU. What do I do next?

Specs:

  • Intel Core i7-10700F 2.9 GHz 8-Core Processor
  • Deepcool GAMMAXX L240 69.34 CFM Liquid CPU Cooler
  • Gigabyte Z490 AORUS ELITE ATX LGA1200 Motherboard
  • Kingston HyperX Fury RGB 16 GB (1x16GB) DDR4-3733
  • Samsung 970 Evo 500 GB M.2-2280 NVME Solid State Drive
  • Asus GeForce GTX 970 4 GB Video Card
  • SeaSonic FOCUS 750 W 80+ Gold Certified Fully Modular ATX Power Supply
 
Aug 12, 2021
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UPDATE: After trying all those things, I disassembled and reassembled everything, same problem. Then I switched out the AIO for the stock cooler, no change. Then I cleaned the RAM and put it back in. Alas, still red CPU light. My final option was flashing the BIOS, I got the latest version from the gigabyte website, removed everything but the PSU and the motherboard, put the usb stick in, started flashing using the Q-Flash option. After that, I tried booting once more, it didn't work. I took the displayport cable out, switched it for an hdmi adapter hooked to a VGA cable and used my old monitor, then I turned the PC on once more. It kept turning on and off, then after doing that for five or so times, it turned on!

I'm really not sure what the issue is, though. I've used DVI instead of displayport in my previous attempts, so the displayport cable is not the culprit. I don't think it's the monitor, it's not showing any issues. I'm still quite confused. The stock cooler is still installed, as it was such a pain keep removing and reattaching the AIO everytime I wanted to try something. If anyone has any idea what the problem really is, please help me out.
 
Aug 12, 2021
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So I swapped the AIO for the stock cooler today, and reconnected the main monitor using displayport. Everything was fine for a few hours then it happened again! This time I disconnected the main monitor immediately and it turned on after restarting using only the VGA monitor (connected with the hdmi to vga converter). Then I tried using only the main monitor with displayport and everything been fine since then. Is it the dual monitor setup that's the problem?
 

revodo

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Jun 10, 2021
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Red LED error light on newer motherboards are pretty straightforward. A red CPU LED light means the problem is with your CPU.
 
Aug 12, 2021
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Red LED error light on newer motherboards are pretty straightforward. A red CPU LED light means the problem is with your CPU.
I thought CPUs just stop working instead instead of not working then working just fine again. Is there a chance that it's the motherboard? Again, if either one has a problem, wouldn't it just not work at all?
 

revodo

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Jun 10, 2021
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I thought CPUs just stop working instead instead of not working then working just fine again. Is there a chance that it's the motherboard? Again, if either one has a problem, wouldn't it just not work at all?
Not necessarily. A CPU can start to have problems before it completely dies. It's possible it's the motherboard, but my bet would be with the CPU. Have you re-applied thermal paste to the CPU?
 
Aug 12, 2021
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Then I would really assume it was the CPU. If you have the money, buy a new CPU and test it, if it doesn't work, send it back or return it.
Yeah it happened again now. It seems to happen after the PC goes to sleep. At least both the CPU and motherboard are under warranty.
 
Aug 12, 2021
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PC didn't turn off overnight, problem only occurs when PC goes into sleep mode. Any thoughts?

EDIT: Tried shutting down the PC, then after pressing the power button same thing happened again, so I turned it off, switched the monitor, and it worked again. I've now reconnected the other monitor and it's up and running. I didn't think I would encounter any problems after disabling sleep mode, but this time simply shutting down and turning it back on didn't work.

EDIT 2: Yep, after shutting down it doesn't turn on unless if I disconnect the displayport monitor. I've tried disconnecting the other monitor, but the red light stays on. The red light only turns off and the pc boots normally if I disconnect the dp monitor! I don't get it.
 
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Aug 12, 2021
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My PC just powered on normally after shutdown for the first time in a while now. I just switched from displayport to DVI... Is this a thing?

UPDATE: Yep, I think it's the DP cable having 20 pins instead of 19 or something.
 
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