Diagnosing a computer that won’t POST

Today, I was asked to diagnose a computer that wouldn’t POST over the phone. POST is the power-on self-test – it is self-check cycle that the computer goes through before ‘starting’ – it usually checks the motherboard (for short circuits), CPU, RAM, and graphics card for some basic level of functionality. Basically, this computer would power on – the lights would come on, power LED remained on, and fans started , but the keyboard was inoperative, as well as there being no display or beeps; additionally, the hard drive LED remained lit.

Since I did not have access to the machine during my diagnosis, I provided the following procedure for diagnosing the problem.

The best approach I can suggest to this problem is to eliminate all unnecessary parts, and try to start the computer with a minimal setup. Reasonably, this cannot be a software problem – no software, except for the BIOS (technically firmware) can cause a computer to not POST.

If at any point, when turning on the computer anything occurs (beeps, noises, things displaying on the screen, lights flashing, etc), please note them.

If at any point the computer does display anything, do NOT continue with the procedure – put things back the way they were, ONE item at a time, and try to turn the computer on between each item. Fully turn the computer off (either by unplugging the power cable from the back, or using a power bar, and waiting 15 seconds), between attempts.

Keep in mind, that the computer will not function with things unplugged, but it can still POST – for instance, without the hard drive, you will not get into Windows, but the BIOS should display an error at least, that will constitute progress, and then you can plug back the remaining components.

It is easy to carelessly damage something inside a computer – ensure that you are grounded (touching the case, or something metal) to prevent static electricity from harming the electronics, and take care when plugging and unplugging things. Don’t force anything to go where it shouldn’t go, and remember where you have taken things out from (and the orientation of the plugs).

  1. Ensure that no USB drives are plugged in, that no floppies are inserted, and that no CDs/DVDs are in the drives.
  2. Ensure that all plugs (at the back) are firmly plugged into the computer. I would suggest also pulling the power plug for a moment (15 sec) and then plugging it back in.
  3. Try to turn the computer on. (We don’t expect anything to happen here)
  4. Turn off the computer, and turn the power bar off as well.
  5. Open the computer case, visually inspect the motherboard (just ensure there is nothing that shouldn’t be there – insects, excessive dust, etc).
  6. Firmly push in all plugs – that includes the power cables (going to the motherboard, and going to the hard drives, dvd drive, and floppy drive), as well as the data cables running to the above components.
  7. Ensure the RAM is firmly seated (see if it is loose, and push it down firmly)
  8. Ensure that the heat sink on top of the CPU is firmly in contact with the CPU (just push it down firmly).
  9. Try to turn the computer on (We don’t expect anything to happen here).
  10. The problem you have is a ‘failure to POST’ (that means the computer has power, but is unable to pass the power-on self-test). This generally results from a problem with the power supply (PSU), CPU, memory (RAM), or motherboard (MB).
    1. We can’t easily test the MB (it either needs to be done from an electronic perspective, or by replacing all the critical components: CPU, RAM, etc). You can visually check the capacitors around the CPU, if any of them do not have completely flat tops, those might be a problem.
    2. We also can’t easily test the CPU (the easiest way is to plug the CPU into another motherboard)
    3. The PSU provides 3 different voltage ‘rails’ – 12V, 5V and 3.3V – you can use a voltmeter to measure that these rails are all functional, and that the voltages are reasonably accurate. On the main MB power connector, the black wire is usually ‘ground’, the orange is usually 3.3V, the red is usually 5V, and the yellow is usually 12V.
    4. The RAM is usually the easiest to diagnose, and quite a likely problem. Each stick of RAM (called a DIMM), needs to make contact with the slot it is in. Humidity and other climatic conditions can cause slight amounts of oxidation that could be the problem, or one of the DIMMs might be faulty.
  11. Pull out all the DIMMs (usually 2), clean the contacts using an eraser, and wipe of any debris from the contacts. Blow out any debris that might be present in the slots to ensure good contact. Try each combination of DIMM and slot – ensure that the DIMM is firmly seated and that the clips snap into place.
    1. Reseat one DIMM in the primary slot. Attempt to turn on the computer
    2. Remove the DIMM, insert the other DIMM into the same slot. Attempt to turn on the computer
    3. Remove the DIMM, insert into the secondary slot. Attempt to turn on the computer
    4. Remove the DIMM, insert the other DIMM into the same slot. Attempt to turn on the computer
  12. If the computer still has made no progress at this point, we are going to start removing unnecessary components:
    1. Unplug the mouse, keyboard, phone lines, audio cables, network cables, and everything else, EXCEPT the video and power cables from the back.
    2. Unplug the power and data cables (usually either one is enough, but we will go with both to be on the safe side) from the optical drives (CD/DVD), as well as the hard drive and floppy drive. Have only one DIMM in the primary memory slot (leave the secondary memory slot empty). Take note of where things are plugged in so that you can put them back where they go later. Most plugs will only fit one way.
    3. Attempt to turn on the computer
  13. Unplug all non-critical PCI cards (usually, all of them – modems, sound cards, usb cards, etc) [At this point, you still have NOT plugged in any of the cables removed in the previous step]
    1. Attempt to restart the computer
  14. By this point, we have essentially eliminated any non-critical component – a computer should be able to POST with just a PSU, MB, CPU, one DIMM of RAM, and a video card (integrated or other).
    1. You also are fairly sure that the RAM is not the problem, as it is usually unlikely that two DIMMs fail at the same time.
    2. The problem is therefore, likely, the CPU or MB (or PSU, if you didn’t test that) – and in this case, you will probably need someone else to look at your computer and see if they can narrow it down a bit more.

One other thing that might be of merit, is to remove the battery, and short the jumpers to reset the BIOS – if you have custom BIOS settings though, they will have to be reconfigured if you get the computer to POST.

Edit: Just a quick follow-up, the issue, contrary to what Occam’s razor might suggest was two fold the computer wouldn’t POST due to a RAM issue (mis-seated/dirty contacts); there was also a loose hard drive cable (different, unrelated problem that just happened to occur simultaneously).

By cyberx86

Just a random guy who dabbles with assorted technologies yet works in a completely unrelated field.


  1. After pulling the power cord, pressing the power button will bleed down the capacitors in the power supply allowing you to work on the computer. The leds will flash indicating power supply is drained.
    Changing the bios jumper while mobo is powered up may blow the board exp. on Dell computers.

    1. A very valid suggestion, thanks. Some motherboards I have worked on also have an LED directly on the motherboard that remains illuminated for a few seconds (15-20) after the power plug is removed. I have never considered it in too much depth, but it would not seem unreasonable that the LED going off corresponds with the capacitors discharging.

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