This is usually caused by the motherboard shorting out. If any part of the motherboard touches metal other than where the mounting posts go then the motherboard will not post. Sometimes it is caused by incorrectly connecting the power supply using the wrong plugs or the addittional power line for the processor.
Unfortunately to test the motherboard you have to take the motherboard out of the case and test it. You place the motherboard on cardboard and connect the power supply with a processor and if you get BIOS beep codes it is working. It would normally give you the code for no ram or no video. You have to connect the two pins momentarily that make the computer turn on (See Motherboard Diagram in manual).
Sometimes a post comes up in a place with no mounting hole causing the short.
It couldn't hurt to give this a try, so I removed the motherboard. I checked the standoffs and they were all in the right places and there were no extras (didn't really think this was it as I spent a good deal of time to be sure those were right the first time). Next I checked that a screw didn't get left in the case as I dropped a couple trying to secure the motherboard but there was nothing extra in the case.
Next, I did as you suggested and placed the motherboard on cardboard and attached the power without ram or video. I got the three beeps for memory. (I only had it on for a little bit as I had not put the CPU fan on it yet)
Not knowing where to go next I put the ram, CPU fan and video card into the motherboard. I started with only 3 of 6 dimms and tested. Got as far as the missing OS as I never connected the drives. I then went to all 6 dimms and got back to the missing OS.
I am not sure what this shows as I was able to boot the computer in the past and it still stopped working later.
Is it possible that the CPU fan was not connected properly and over heated causing the system to shut down and result in an 1-5-4-2 error? If it remained hot could it just return this error or should some other code be returned that would elaborate on this error?
I only bring this up since, with all the other fans, it is not obvious that this one was running.Since its connector is off in a corner of the board it is possible that I didn't get it on fully or jostled it when puting on that fan hood/tunnel.
Later today I will reassemble the computer and see how long it lasts this time making sure all of the fan connectors at secured properly.
Other than this theory I am not sure what else to look at
Any other ideas would be appreciated.
It is very dangerous to connect any processor without a cooling fan, even for a second. In most cases the motherboard will not work because the fan header has a failsafe which shuts down the motherboard immediately if the 3 pin CPU fan header does not have a fan plugged in. This is a generic statement not meant for any specific motherboard - not all motherboards have this kind of failsafe, but it is very common. If you get beeps you at least know the Motherboard is working at some level and the BIOS is functioning.
Next would be to connect other components one at a time to try to determine what was not working. I would maybe put in the memory and the video card and see if you get a signal to the monitor. If one of the I/O cards like the Video Card is not seated correctly or is incompatible you can get a message or maybe even no video.
Hopefully when you mounted the motherboard you used some offsets that raises the motherboard above the mounting plate so the circuits do not touch the metal mounting plate inside the case. I only mention this because I did not want you to assume otherwise. Occassionally someone tries to just put the screws in without the raised posts. They maby be built into the motherboard or they may look silver or brass.
The Main power supply on newer motherboards may be a 24 pin variety. Some older boards used 20 pin power supply main connectors. You probably need a 24 pin power supply for most newer motherboards. You can actually count the connectors. You cant just get a converter plug, you have to use the type of power supply designed for your motherboard. Power supplies vary in quality. I think last time I used a Corsair model, but in the past I have also used Antec. The wattage on the power supplies are crucial for higher end video cards. Some power supplies like Deer should be avoided. This is one area you can select the cheap models or buy a power supply less then the specifications. If anything you need to exceed specifications.
Be careful, you dont want to burn up the processor. That is the way of pain. Also make sure the wattage and the requirements of the RAM meet the motherboard. Some of the X58 Chipset motherboards have varying wattage or voltage requirements for the RAM.
Hopefully this link will work.
This is a good article about DDR3. It makes for interesting reading. Keep in mind that some motherboards require you use RAM that is rated at a specific voltage or have a voltage limit. My Intel DG35EC motherboard could only use DDR2 800 ram rated at 1.8v or some such rating. As long as you used the correct RAM for the motherboard it would run fine. However, some RAM manufacturers would try to sell 2.0v DDR2 which might be considered overclocking the RAM to get higher speeds. With Intel Motherboards you have to pay attention to the Motherboard Compatability requirements. I would call this Intel's dislike of overclocking or strict adherence to their specifications and what is considered Industry Standard Specifications.