I recently bought all the components and put together my PC, it turned on for about 3 seconds, lights on, fans spinning then turned off. I had to turn off the PSU inorder for it to turn on again for 1-5secs. Now I tested it with a new motherboard, exact same make and model and tried it with a new powersource of the same wattage (750W). But it still won't work, I can power on the PSU with everything attached but the motherboard. Once I plug in the 24 pin or the 8 pin the PSU will not turn on. I have no idea what do try next and have read tons of articles and forums saying its just the Mobo or replace the PSU.
For a new system to turn on for only 3-5 sec suggests to me three possibilities:
1. You could have a short from mobo to case because of the way you have mounted the mobo in the case. To test this, go to "breadboarding". This means you take the mobo and all components out of the case, and place the mobo on an insulating surface on the table. You mount / connect to the mobo ONLY the bare essentials first: CPU with heatsink and fan, PSU conected, ONE stick of RAM (check the mobo manual for the correct slot to place only one stick), keyboard, and monitor connected to video output. IF your mobo has built-in video, use it and do NOT install any extra video card. But of course, if you have no mobo video, you must install your video card and make sure it has any power supply required plugged in from the PSU. ALSO ensure that the power supply connector for the CPU itself, as well as the main one, is plugged in. Ensure the CPU cooling fan is connected. Now, turn on power and see whether it will complete the POST until it tells you there is no device to boot from. Maybe reboot and enter BIOS Setup to look around, but don't change anything yet. If the system will power up this way, it is basically OK.
Now you can add more system components one at a time. Try adding in all your RAM first and ensure that works. Next add a HDD and see if it still works, even though there is no OS on the HDD to boot from. All this will establish whether the hardware is OK. If yes, disassemble and mount back in the case. This time be VERY sure you have mounting standoffs ONLY where the mobo has matching holes and NOWHERE ELSE. Make sure there are no loose pieces rolling around. Here's an odd one I ran into once: I discovered that, after mounting in the case, there was a springy metal finger at the back (where the mobo ports poke out of the rear cover plate) sticking inside one connector port, instead of being outside of it.
2. Even with good hardware you can get 3 to 5 seconds to turn-off if the CPU cooling is not working, because the CPU WILL overheat to the point that the BIOS shuts down the system to protects it if cooling is poor. Most common causes of this are:
(a) poor physical connection of heatsink to CPU because the heatsink mounting system is installed incorrectly, resulting in misalignment or inadequate contact pressure; or,
(b) poor, or no, or too much thermal contact paste applied between heatsink and CPU. Did you follow the manufacturer's instructions on how to install? If your heatsink came with pre-installed paste, did it also have a plastic film over it that you failed to remove?
The third cause is obvious - if you do not have the CPU cooling fan connected and it does not turn at all, you'll see that. But this normally will take longer than 5 seconds to cause an overheating problem.
3. Last item that could do this is the way your CPU fan speed is monitored. Some BIOS's (not all) have an extra protection system to check CPU cooling. The main system all have is to measure the actual CPU internal temperature with a sensor that is part of the CPU and is monitored by the BIOS. The extra system in some BIOS's watches the CPU cooling fan speed signal (fed from the fan motor to the mobo in the wires from the fan that you plug into the mobo's CPU_FAN port. You DID plug it in there, right? If the BIOS fails to see a good fan speed signal within a few seconds of power-on it will shut down the system without waiting for the CPU temperature to rise. (Some BIOS's do not have this feature). So IF your CPU fan is not turning, or IF there is a poor connection that fails to get the fan speed signal back into the mobo connection, you could see a shut-down for this reason.
By far the most common mistake by new builders that can cause this is in the placement of the mobo standoffs. The case has several (probably about 15 to 20) holes that are threaded that MIGHT be where your mobo needs support, but NOT all are to be used. To support your mobo you must install in just the right holes little standoffs. The common form of these is a piece of brass about ¼" long with a threaded short shaft out one end, and a threaded hole in the other end. You screw these into the right holes of the case, leaving threaded holes showing on their ends for the screws that hold the mobo in place.
Where do they go? TEMPORARILY place the mobo in the case as close to its final position as you can. Use the back port panel as a guide. Now note carefully exactly where the holes are in the mobo for fastening it down. Often each looks like it has little metal star arms sticking out around it. These are Ground connections, and the screws you put in there are supposed to connect to the case to ground the mobo at those points. BUT there should never be any other places on the mobo that touch the case. Typically there are 8 or 9 holes in the mobo for these screws - three down the back edge, 3 in a line down the middle, and 2 or 3 along the front edge. Note where they all are. Now remove the mobo and place your standoffs in those case holes. Re-locate the mobo in the case temporarily and check carefully that every one of the holes has a standoff under it, and there are NO standoffs in places with no matching holes in the mobo. Move any that need it. It IS acceptable to have a mobo hole with no standoff under it (and hence no fastening there) if the case does not have a threaded hole in which to place a standoff, but you don't want too many of those unsupported locations.
When you have all the standoffs placed correctly, remove the mobo again and prepare for final installation. Place the rear port panel (a custom one should have come with your mobo) in the back opening for it and snap it into place. Probably a good time to mount the PSU in the case. Anything that needs to be mounted on the mobo now should be done. This is especially true for the mounting system for the CPU cooler - sometimes you must mount a custom backing plate on the back in place of the one that came pre-installed. I have found it easier sometimes to mount the CPU, its heatsink and maybe its fan on the mobo while outside the case - you can get better access to these parts that way, and sometimes they need careful but strong force to fasten them down straight.
When you do mount the mobo in the case, check the way the back ports line up with the custom cover plate. Place all the mounting screws in their holes but don't tighten any of them until they are ALL in place - sometimes you need to shift the mobo slightly to line up the holes with the standoffs underneath. Tighten when they're all in. Now you can connect the power supply lines from PSU to mobo (don't forget the one for the CPU) and the video card, if there is a connection required. Make sure the CPU cooling fan is connected to the CPU_FAN port. If your mobo has a fan connector labeled PWR_FAN, it normally is used ONLY for a special connector that looks like a fan connector on wires but comes out of the PSU, not from a separate fan. If there are no such leads from the PSU, just don't connect anything to that mobo connector.
Just to check, lift up the case carefully now and turn it over and around in the air, listening for any loose parts sliding around. Find any and get them out. Now proceed with the rest of your major components - RAM modules, HDD's into drive bays, etc.
NOTE one thing that can get done wrong easily. You MAY have front panel connectors for USB and / or Firewire (aka IEEE 1394a) ports, and these connect to the mobo by cables that end in 9-pin connectors (they look like 2 x 5 pins, with one pin missing). The trouble is that the SAME connectors are used for both types of ports and cables, and yet mixing them up can cause permanent damage to your system when it is used! So be VERY sure that you plug the USB ports cable into the USB connection point on the mobo, and the Firewire cable into the Firewire mobo connector.
Glad to hear your hardware is all good. Have fun re-installing and let us know how it goes.