While there are plenty of references about PC diagnosis on the internet, I have recently run into a problem that seems less common, when repairing someone’s PC.  The symptom is: upon powering on the PC, it will turn on (in terms of power LED and CPU FAN, but no power-on self test (POST)) for a while, turn off, turn on for a few seconds, turn off, and so on so forth.

Google searches did not turn up too many references on this symptom, perhaps due to the keyword similarity of this symptom to two other entirely different but more common problems.  So I decided to write about my experience, but first, let’s summarize the two other common problems first:

  • PC turns on for very few seconds, then immediately turns off (and stays that way) – from my experience, if you have just assembled a PC from new parts and run into this, you likely have a short circuit or some hardware is not connected properly – also check the CPU FAN connection.  If a PC that has been working for months suddenly behave like this, the power supply unit (PSU) may have failed.  A motherboard failure is also a possibility, but personally I have encountered failing PSU 10 times more than failing motherboard.
  • PC turns on a few seconds, turn off, then turn on again and proceed to boot up the operating system normally – this is actually almost a normal (but annoying) behavior of some motherboards.  In some cases there may be BIOS settings or BIOS upgrades to cure it.

I have seen enough PSU failures to know that the “on, off, on, off, etc.” is not a usual symptom of PSU failure.  It is more likely to be a motherboard failure.  However, the only reference I found from google found the cause to be a defective CPU.  Anyway, no matter what my instincts say about the root cause, I followed the systematic way of PC diagnosis:

  • I disconnected everything non-essential for POST, including all drives.
  • Since the PC I’m repairing has 2 modules of RAM installed, I tried to boot it up with a single module of RAM, and tried each of them separately.
  • Failing to POST, I replaced the PSU (I always have at least one spare PSU at home, at times I even had two)
  • Failing to POST, I removed the display card (on a system without display card, it would beep)
  • Failing to beep, I replaced the CPU (luckily I happen to have a spare CPU for this socket)
  • Still failing, the only remaining faulty component has to be the motherboard.

So my instinct is right this time.  I consider myself lucky in not experiencing many motherboard failures, because a motherboard is most troublesome to replace.  The failed motherboard is Asus P5K SE.  Looking back, the choice of this motherboard was a mistake.  I have always been a fan of Asus motherboards, but I also knew that the value motherboards may have hidden lower quality components that are not stated in specifications, i.e. there was a USD25 difference between a regular motherboard and a value one, with the only difference in specifications being additional I/O ports that most people don’t need, and/or proprietary features that are unimportant.  I should have bought the regular P5K instead of the SE value version.  (In addition LE, LX, or X may denote a motherboard is of value class.)

P5K SE is a socket 775 motherboard.  To replace it, there aren’t many socket 775 motherboards to choose from in year 2013.  Most of available ones I found are based on G41, but not including Asus.  So I purchased Gigabyte GA-G41M-Combo rev. 2.0, instead of MSI and Asrock which are also available for my purchase.  Perhaps I may write a mini review of this Gigabyte motherboard soon.

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