Question No POST, Error 23 Asus Maximus Hero - can't figure out if CPU or MOBO | Old Machine ?

[commissar_mo]

Appreciate any help in advance - I know these are super tedious hardware problems to help people with!

Mobo: Asus Maximus Hero VII ROG (2015)
CPU: 4790k 4GHz stock (2015)
Cooler: Corsair H80 (2015)
RAM: Corsair Vengeance 8GBx4 (2015)
PSU: New Corsair 750W (2021)

Machine had recent new install of Win10; no issues. I suspect the 'energy saving' feature messed up the BIOS (it seems buggy, usu never use it, did not disable it, would sometimes hang and not restore requiring hardware reset).

One day machine would not POST. Error 23 on the motherboard. Apparently no one (including Asus, I asked them) knows what Error 23 actually means - people online have reported conflicting things, Asus told me it's "maybe CPU related." The exact same hardware has definitely had some issues over its 6 year lifespan, but it was running without issue for 1 month following the new OS/SSD install.

1. I stripped all components to the above parts - CPU, Mobo, RAM.
2. MOBO starts, fans go, I still get ERROR 23, no post, no signal coming off the onboard GFX (or the GPU when I plugged it in)
3. CPU inspection shows some lightly black contact pads in 2 small spots (6-7mm diameters), not sure how long that was there; I have read it can be from arcing from unbalanced pressure applied. But the CPU is 5+ years old and has run in the same board with the same cooler/same mount for years so I don't think it has that kind of damage.
4. I bought a used CPU which is almost the exact chip (4790 3.6GHz (not the 'k' unlocked version)) to test. Swapped CPU and now the MOBO will not power on (powers on for less than a second and shuts down, clearly some sort of protective feature being triggered). Reset CMOS, removed battery, waited, battery left off. MOBO will still not start at all.
5. So it powers on with old CPU >> Error 23, No Post. Doesn't Power on AT ALL with new replacement CPU.

What could cause this??

6. Have ordered legacy replacement MoBo, exact same; enroute. Will swap out to see if MoBo issue to blame.

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My Thinking/Next Steps:
*I have not messed with the BIOS because 1) BIOS has been the same since 2015 and I had no issues. Machine also ran for 1 month after new OS install. IF BIOS got corrupted by the Win10 power saving feature, new Mobo should fix this right? OR do I need to flash the new Mobo also?? I just can't see how BIOS will be an issue as I'm using 2015 same-era parts.

**I don't have all the Mobo screws (I haven't for years) and so there's only 3/7 holding the Mobo to the case. I've heard rumored occasionally this can create a short, but I've never once had that issue, and the machine has run for 300+ days in the past, so I've discounted it.

***The CPU cooler seems fine, when I remove the cooler housing off the CPU (but keep its pins plugged in to trigger recog), nothing changes.

****My current guess is the Mobo may be the underlying issue, but I am very worried when I replace it I'll have the same issue and won't know what to do.

 

[commissar_mo]

Unfortunately did not get any help from the forum probably because the issue was quite complicated, but I solved it and wanted to post for future readers.

So there's still a lot of mystery around Error 23 on the Asus Hero Mobos. I asked a different Asus person and they said Error 23 usually indicates a problem with the CPU. Why the error is not listed anywhere though is not clear. People claim that it has also come up when RAM is not seated correctly.

My theory is that Error 23 is some sort of an electrical contact error - i.e. the Board is recognizing that the CPU and RAM are installed (in a minimal MOBO, PSU, RAM, CPU setup) but there may be contact errors between the chips.

1. Hence some people have reseated their RAM and Error 23 was resolved.
2. A video from Taiwan probably on YouTube discusses how to fix Error 23 (clean the RAM and CPU contacts on the MoBo with 91% ethanol, they show it resolves Error 23.
3. And finally Asus mentioned a functioning CPU but which has contact errors can also cause Error 23 (I wonder does a bent MoBo contact pad for the CPU cause it? Or is that some other error?)

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I wound up, as often has happened to me when failures are tough to isolate, buying a replacement CPU, MoBo, and GPU (I originally stupidly thought it was the GPU - DO NOT do this if you fail to POST - just remove the GPU to isolate the 4 main components + CPU cooler and run off MoBo graphics if you can).

I eventually found out it was a contact fault on the CPU since the new MoBo produced Error 23. (this took a lot longer to find out than it should have by process of elimination... because it turned out the CPU replacement I bought WAS DEFECTIVE UGH and so I was trying to isolate the problem with a replacement that was itself stealth broken - I chose to buy a used CPU even though they still sell this model new).

*I randomly suspected the CPU was defective by examining it under a magnifier when I saw how scuffed up it looked. Turned out a small, what I believe is a capacitor, had broken off the CPU.

**Once I got the replacement CPU, everything worked again and no other components needed to be replaced. Had some BIOS issues (seem to always be BIOS issues...) but eventually Windows 10 booted (luckily didn't get corrupted from the CPU change - I've seen that happen).

***Not really sure how I could have done all of this more efficiently, except maybe had been biased against the CPU earlier.

****Inspection of my original CPU under magnification, around the aforementioned slightly discolored areas, did indeed show small damage to the coating on top of the contact pads. I believe that was causing Error 23.

*As yet another aside - I believe it's possible that the unevenly applied pressure of a cooler can cause potential arcing of the CPU against the contact plate which I believe could cause CPU damage. I'm a bit worried that could have happened here, so will be replacing the cooler (though I don't know if this is realistic as the Mobo plate should keep the CPU locked in place. Potentially THAT is also damaged in this 5yo Mobo...