The 4pin or 8pin cpu harness back into the motherboard is a good start. Strip down to the bare components to see if you get a basic post.
POST troubleshooting steps
The Power On Self Test (POST) is a test a computer takes each time the computer is turned on to make sure all the hardware is properly working before trying to load the operating system. If the computer does not pass POST it will not boot. See our POST definition for further information on this term and related links.
Tip: Make sure the computer turns on, if nothing happens (no lights, no sound, no fans, etc.) the computer has a power related issue.
Caution: Some of the steps below recommend removing physical parts within the computer. While in the computer it is highly recommend that you be aware of ElectroStatic Discharge (ESD) and its potential hazards.
Remove new hardware
If any new hardware has been recently added to the computer, remove that hardware to make sure it is not the cause of your issue. If after removing the new hardware your computer works it is likely the computer is either not compatible with the new hardware or a system setting needs to be changed to work with the new hardware device.
USBRemove any disks or USB devices
Remove any disks, CD's, DVD's that are in the computer and if any USB devices (iPods, drives, phones, etc) are connected disconnect all of them as well. Reboot the computer and see if anything changes.
Disconnect external devices
Remove everything from the back of the computer except the power cable. Turn on the computer and see if it beeps normally. If the computer has never beeped keep the monitor or display connected to see if any change occurs.
Identify beep code
If you are receiving a sequence of beeps see the beep code page for a listing of different beep codes and their explanation or your motherboard or computer documentation. These beep codes are meant to help identify what computer component is failing or bad. If your beep code is not listed, continue troubleshooting.
Check all fans
Make sure all fans are running in the computer. If a fan has failed (especially the heat sink fan for the CPU) your computer could be overheating or detecting the fan failure causing the computer not to boot.
Check all cables
Verify that all the cables are properly connected at that there are no loose cables by firmly pressing in each cable.
All disk drives should have a data cable and power cable connected to them. Your power supply should have at least one cable going to the motherboard. Many motherboards may also have additional cables connected to them to supply power to the fans.
Disconnect all expansion cards
If the above recommendations still have not resolved the irregular POST, disconnect the riser board (if applicable) and each of the expansion cards. If this resolves the issue or allows the computer to POST connect one card at a time until you determine what card is causing the issue.
Disconnect all drives
If you were unable to determine by the beep code what is failing or do not have a beep code disconnect the IDE, SATA, SCSI, or other data cables from the CD-ROM, hard drive, and floppy drive from the Motherboard.
If this resolves your irregular POST or generates error messages re-connect each device until you determine what device or cable is causing the issue. In some situations it can also be a loose cable connection that causes the issue.
Remove the RAM
If you continue to receive the same problem with all the above hardware removed, disconnect the RAM from the Motherboard and turn on the computer. If the computer has a different beep code or if your computer was not beeping and is now beeping turn off your computer and try the below suggestions. Making sure to turn off the computer each time you are adding and removing the memory and then turning the computer back on to see if the suggestion resolves the issue.
Re-insert the memory into the same slot.
If you have more than one stick of memory remove all but one stick of memory, try rotating through each stick.
Try one stick of memory in each slot.
If you are able to get the computer to boot with one or more of the sticks of memory it is likely you are dealing with some bad memory. Try to identify what stick of memory is bad and replace it.
If you are able to get memory to work in one slot but not another slot. You are motherboard is defective you can either workaround the issue by running the memory in a different slot or replace the motherboard.
Power cycle the computer
In some situations a computer may have power related issues often caused by either the power supply or the motherboard. To help determine if this is the cause of your issue try turning the computer on, off, and back on as fast as possible, making sure the computer power light goes on and off each time. In some situations you may be able to temporarily get the computer to boot.
This should only be used as a temporary workaround if you are able to get this to work. Often this is good for users who may have not done a backup and need to get the computer up one more time to copy files before starting to replace hardware.
Disconnect and reconnect the CPU
For users who are more comfortable working with the inside of their computer or who have built their computer one last recommendation before assuming hardware is bad is to reseat the CPU by removing it and putting it back into the computer.
Loose BIOS chip
If your motherboard has a BIOS chip, it can become loose over time due to heat expansion and cause the computer to give an irregular POST. Gently press down on the BIOS chip to make sure it has not become loose.
Bad motherboard, CPU, RAM, or power supply
If after doing all of the above recommendations you continue to have the same issue unfortunately it is likely that you have bad Motherboard, PSU, CPU, or RAM. The next step would be either to replace these components or have the computer serviced. If you plan on doing the repairs yourself or you are a repair shop it is suggested that you replace the Motherboard first, RAM, CPU, and then power supply in that order or try swappable parts from other computers.
Info was provided by
http://www.computerhope.com/issues/ch000607.htm
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