It was the rear panel bios flashback button that was being held down by the rear IO panel. You don't have a separate rear IO panel so it's probably not the issue, but just check to make sure the button on the rear panel feels like it's spring loaded and not stuck depressed.
On another note.... what's the cmos reset on that? Is it just a pair of pins that you short out, or is it 3 pins that you move a jumper over on? If it's 3 pins, make sure you don't have the jumper on the wrong two pins.
@sswilson Ah, thanks for clarifying regarding the flashback button. I don't think it is stuck depressed but will double-check that when I get home. Of note, I did use that button when I updated the flash so perhaps it stayed stuck or partially stuck.
The CMOS reset is the two-pin style that I short out with the tip of my screwdriver.
A colleague of mine who builds PCs for a living suggested that sometimes RAM issues (even if I am not getting a RAM warning LED) can potentially cause the types of power-up problems I'm having. So I will double-check RAM as well. However, I'd need to remove my CPU cooler to do that since the cooler sits above them.
@JD – Sorry, that's a brain cramp on my part. The board is Gigabyte and the GPU is Asus. And I definitely flashed the correct BIOS (fair question, though! LOL). And yes regarding CPU power. Per your earlier request, I'll attach some photos below.
Current update: The Ethernet / WiFi problem has mysteriously gone away. No idea why, though I did swap ethernet cables today – though why that also fixed the WiFi issue is beyond me (WiFi stayed fixed even after I removed the ethernet cable). Ethernet / WiFi functionality remained working even after a full power-down and re-boot cycle, so that is encouraging.
With Internet available, I downloaded Gigabyte Control Center and through it all the board-related drivers. The Performance panel of GCC doesn't show any issues. I'm able to control fan speed, RGB colours, etc. I was also able to re-start Windows without problem.
The power-up problem remains, though. I have to reset the CMOS to get it to power up. One oddity, though: on my last power-up attempt the system (pre-Windows) displayed a new 'boot failure' screen that I hadn't seen before. I selected the top option and it booted into Windows without any problems. Previously, it either went straight into the BIOS or straight into Windows. FYI, the 'optimized defaults' is the preset that I was referring to previously.
Here's the 'boot failure' message:
And here's some photos of the motherboard and rear of the case. I've downsized these to 1024 px wide. Let me know if any would be useful at full resolution. I'm happy to take additional / zoomed-in photos of anything else if it would be helpful. Please excuse my sloppy cable management – wasn't great to start with and then I've made it worse by my fiddling as I try to check everything (at least, that's my excuse and I'm sticking to it!).
@Bond007 – Thanks for the suggestion. So far I haven't tweaked anything. Everything is just as-is. I'll try toggling the XMP/Expo setting and see if it does anything. I"m trying to track down a new CMOS battery as well just in case.
Those CPU power cables should split apart, but if you're certain the extra 4pin isn't touching anything then I suppose it's okay.
When installing the heatsink, did it seem like it was making contact with anything beside the CPU itself? Kind of seems like there's a short somewhere since the behaviour seems to change as you move cables around on the I/O area.
Noticed the CPU fan header doesn't have anything plugged into it either, are you just relying on the case's fan hub entirely?
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