[SOLVED] No POST, CPU error LED lit. Already changed mobo and CPU.

[TFBeyond]

I'm at my wit's end with this one.

PC behaves entirely normally for between 1 and 10 hours or so before abruptly powering off. After it has abruptly powered off, whenever the power button is pressed the system fails to reach the POST screen while all fans go at maximum RPM and the red CPU warning LED on the motherboard lights up. This state persists until the PC has been left idle and unplugged for several hours.This build has been entirely functional and normal for nearly 3 years up to this point.

I have already changed out the motherboard (originally it was a Gigabyte Z390 AORUS MASTER, but I purchased a MSI MPG Z390 Gaming Plus to see if that fixed the issue. Naturally, this means I have done the following:

• Reseated motherboard, CPU + Cooler, RAM, GPU, all power connectors.
• Reapplied thermal paste

When this failed to remedy the problem, I RMA'd my i7 9700K for a new one, but this has also failed to solve the problem. This means I am also confident that the fault is not owing to CPU failure or bent pins.

Watching temperatures whilst leaving the PC running overnight under moderate load showed that no temperature exceeded 70C.
I have tried disabling any overclocking to return the CPU to the standard 3.6GHz speed, no change.
I have tried changing which PSU lead provided power to the CPU. No change.
I have tried changing the PSU between multi-rail and single rail OCP. No change.
I have tried thoroughly cleaning the entirety of the case to ensure minimal thermal buildup.

For completeness sake, here is my spec list:

Motherboard: Originally Gigabyte Z390 AORUS MASTER, swapped in MSI MPG Z390 Gaming Plus.
CPU: Intel i7 9700K
RAM: G.SKILL F4-2400C15D-16GTZR Trident Z RGB Series 16 GB (8 GB x 2) DDR4 2400 MHz (currently running at 2133MHz)
PSU: Corsair RM750i
GPU: MSI RTX 2070 SUPER GAMING X TRIO

Any insight as to solutions or further troubleshooting would be greatly appreciated.

 

[Astralv]

Did you try to work it outside the case? Sometime the case may be causing shortage. Run without GPU, try different PSU. Run just 1 RAM.

 

[TFBeyond]

Did you try to work it outside the case? Sometime the case may be causing shortage. Run without GPU, try different PSU. Run just 1 RAM.

I admittedly have not tried breadboarding it yet. I'd be surprised if it was the case, though, given that the system has been running fine for 3 years up to this point and the problem only seems to occur under load (as best as I can tell with the relative runtime before it fails). The fault is also not provoked if the case happens to be nudged or otherwise moved during operation. When the POST failure is occurring, it does so regardless of whether RAM is inserted, how much is inserted or what slots it's inserted in. Same for GPU. Powered or unpowered, inserted or absent, across both boards.

Trying to get the PSU RMA'd to swap that out, given it's 6 years into its 10 year warranty.

 

[Astralv]

I admittedly have not tried breadboarding it yet. I'd be surprised if it was the case, though, given that the system has been running fine for 3 years up to this point and the problem only seems to occur under load (as best as I can tell with the relative runtime before it fails). The fault is also not provoked if the case happens to be nudged or otherwise moved during operation. When the POST failure is occurring, it does so regardless of whether RAM is inserted, how much is inserted or what slots it's inserted in. Same for GPU. Powered or unpowered, inserted or absent, across both boards.

Trying to get the PSU RMA'd to swap that out, given it's 6 years into its 10 year warranty.

In my case, it was the CASE. Computer worked 3-4 years and then all LED lights were red on motherboard and all similar behavior you described. It worked in the new case for a few more years, then I started to have blue screens and endless repair loop was a give up point.