Need help with troubleshooting POST issues

It's been a long time since I've needed to reach out for help and experience here on Tom's Hardware. I'm at a point where I'm really pretty stuck right now, due to inability to test some of my components with other parts. Looking for ideas on what could be wrong, and invalidation of some of my current ideas (help narrowing it down).

System Specs:
PSU: OCZ ModXStream Pro 500W
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341016
RAM: 4x G.SKILL RipJaws 4GB DDR3 1600
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231314
MOBO: ASRock Fatal1ty P67 Performance
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157253
GPU: XFX HD 6870 1GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150521
CPU: i5-2500k
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072

Symptoms:
Power button is pressed, all fans spin, HDD LED and power LED light up. HDDs and DVD-RW have power. GPU fans spin, indicating GPU has power. No POST beep, no debug code is listed, but to compound the problem, the system would not POST beep or list an error code during normal operation. Display does not register input from GPU. Power off requires long press of front power button to shut the machine down, which may indicate that the system has made it past POST.

Troubleshooting Performed:
Unseated and re-seated the graphics card. This has happened before, and removing/re-installing the GPU had resolved it in the past. No change after doing this.

Swapped GPU for a known working GPU. No difference in behavior.

Unplugged/re-plugged 8 pin CPU, 24 pin ATX and GPU power. No change.

Connected the monitor to the GPU via HDMI to rule out display input failure.

Checked motherboard screws to make sure they weren't too tight and possibly causing a short.

Checked motherboard caps and components for physical defect - I didn't seen any swollen or burst caps, and all exposed components looked fine.

Checked CPU for physical problems - none observed. Each contact had a single pin impression, socket looked good on the motherboard. Since I was getting no POST or debug, I thought it possible that if the CPU isn't available for instruction, the motherboard may not get feedback on errors. The mobo should be able to handle that without a CPU, but it was worth a shot.

Removed memory modules to try and force a POST failure with an error code. Behavior did not change.

Cleared CMOS with the button on the motherboard, as well as removing the CMOS battery.

Known Issues/notes:
My kids use this machine primarily, though it's all hardware from my computer, which I upgraded a month ago. They complain of mouse and keyboard input freezing up at random, which requires a reboot to get working properly again. Seems like the mouse may have been causing it, since it hasn't been an issue for around a week since swapping it out, but still worth noting.

Some of the 3-pin fan headers on the motherboard have never worked, and I didn't deem it a big enough issue when I ordered the motherboard 3 years ago to submit for RMA.

When swapping out hardware, I had a Hyper 212+ on the motherboard that used to be in the machine. During the swap, I misplaced one of the mounting screws, and was unable to use the CPU cooler, so I used the stock cooler that came with the i7-4770k I upgraded to on the i5-2500k in the socket. Temps seemed to be in-line during normal use, never going over 75c or so, and I hadn't found time to swap back to the Hyper 212+ after finding the mounting screws, so this is still the cooler in use. Because I don't see any defects on the CPU, I'm assuming it didn't cook itself.

What I haven't done (yet):
Test GPU with different display.

Swap out the PSU for a known working unit. (I will need to purchase a new PSU to test this, or yank my working TX-750 out of my machine and undo an hour or so of cable management...probably less pain just to buy a new one)

Swap out the CPU for a known working unit. (I have an i7-4770k, but it's not compatible with the motherboard - this would be a new order of a "cheapie" throwaway 1155 socket compatible CPU to rule out the issue, non-ideal.)

Remove all unnecessary devices from motherboard (HDDs, DVD-RW).

Completely remove motherboard and inspect standoffs/solder pad around screw holes to rule out some sort of short.

Use multimeter to check voltage on PSU power connectors. (I'll have to obtain a multimeter to do this - used to be able to borrow one, but I've since moved and don't know anybody with one.)

Summary:
System does not appear to be passing POST. Troubleshooting attempted and troubleshooting planned listed. Looking for experience with similar issues to help slim down troubleshooting to be done, and possibly other troubleshooting steps to identify areas of failure to rule out. Very open to suggestions!
3 answers Last reply
More about troubleshooting post issues
  1. fan pin, mouse, usb , keyboard not work properly...it's cause by faulty IC I/O
    when IO broken , then you got blank display replace that FINTEK ic, it's should normally again.

    twice i repair this p67 msi with resistor burn beside IO, and rusty leg IO
  2. I'm not too savvy on the I/O IC, same with soldering for that matter. Not even sure where I'd begin there. Are you confident that it's an issue with the motherboard from the description I gave?
  3. Quote:
    My kids use this machine primarily, though it's all hardware from my computer, which I upgraded a month ago. They complain of mouse and keyboard input freezing up at random, which requires a reboot to get working properly again. Seems like the mouse may have been causing it, since it hasn't been an issue for around a week since swapping it out, but still worth noting.

    Some of the 3-pin fan headers on the motherboard have never worked, and I didn't deem it a big enough issue when I ordered the motherboard 3 years ago to submit for RMA.


    from description about your mb issue..yes, i think u'r MB has a faulty IO
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