Mysterious hardware issues at boot

[Jesse_V]

My fight with my graphics cards continues apparently. This time there's some mysterious behavior that I can't explain and I would appreciate help with.

Symptoms:
1. No POST splash screen is shown at startup on either of my two monitors.
2. Neither a 970, nor a 480, nor a known-working 560 TI rev their fans at startup.
3. The system never seems to boot into an OS, since the HDD light never clicks even after I'd expect it to get through GRUB automatically. Of course, the HDD light could be fault, but I suspect not because I tightened all the connections.
4. PWR_FAN and SYS_FAN2 do not rev, but CPU_FAN seems to work, as does the other fans powered by the PSU.

Setup:
The Gigabyte motherboard has two PCI-E slots. I own a GTX 970, a GTX 480, and a GTX 560TI. I have two monitors, which I connect to one of those cards. CPU is watercooled by a Corsair H80, which is connected to the PSU and to CPU_FAN for RPM control. PSU is 750 watt.

Diagnosis:
1. After first noticing that the machine refused to boot normally, first noticed symptoms 1 and 2. Many systems are involved here, so it's hard to successfully diagnose.
2. Switched PSU cables around several times, since the 480 takes two rails and the 970 only needs one, so there were several combinations. No changes.
3. Suspected that I was out of power, so I removed the 480 from the top PCI-E slot and plugged the monitors into the 970 in the bottom PCI-E slot. No change.
4. Switched PSU cables around again. No change.
5. Plugged the 970 power rail into a never-before-used port on the PSU. No change.
6. Removed the 970 from the bottom slot and put my known-working 560 TI in the top slot. Connected monitors, applied power, no change.
7. Switched PSU cables. No change.
8. Removed 560 TI and plugged in to the top slot a tiny 2006 ATI card from a school computer, one that didn't require external power. Connected monitors. No change.

My conclusion at this point is that both PCI-E slots are bad. I really don't see how that's possible for them to both die at the same time, so what else could be at fault here? Symptom 4 seems to suggest that there's a larger issue here with the motherboard or the PSU. What else could I try? Could a fried CPU do this?

[billford]

Have you tried resetting the BIOS?

[Jesse_V]

Have you tried resetting the BIOS?

No I haven't. How can I do that without being able to see anything on the screen? I can do that if I could see the POST, but I can't.

[billford]

You'd need to look at the motherboard manual but there are usually two pins that can be briefly shorted together, or unplug the computer from the power socket and take out the battery for 10 minutes.

[jimerickson]

bad psu is my guess.

[JimF]

Does your motherboard have internal graphics? You may have to select internal or PCIe graphics in the BIOS. Some motherboards can work with both, but others can't. They don't bother to tell you in the specs either; you have to try it yourself.

[Jesse_V]

You'd need to look at the motherboard manual but there are usually two pins that can be briefly shorted together, or unplug the computer from the power socket and take out the battery for 10 minutes.

Good call. I did that, and while I was waiting also unscrewed the PSU, reorganized some cabling, and plugged everything back in. I put the 970 in, and POST happened, so I installed the 480 as well, tweaked the BIOS to my liking, and everything seems to be working well so far. I'll of course post back if the problem reoccurs, but I'm blaming it either on a faulty PSU connection, though I can't yet rule out a bad rail. In any case, the 970 is folding away. I'll add the 480 if I don't get any resets or other issues.

[billford]

I'm blaming it either on a faulty PSU connection, though I can't yet rule out a bad rail.

I'd agree, corrupted BIOS is not impossible but pretty unlikely. Whatever, glad it's working again