Power Supply or Motherboard Issue?

I am assessing a client's PC (HP Pavilion) and upon plugging the power cord into the PSU, I noticed immediately that the small light next to the connector on the PSU was flashing green. As I tried to start up the PC, nothing happened. I mean nothing! My immediate reaction was to begin with troubleshooting the PSU.

My first step was to attempt to reset the PSU. I disconnected the power cord (edit: I also disconnected the 24 pin power, p4 power, and SATA power from all the drives), and switched the small red switch from 115 over to 230 and then back to 115. I then attempted to reconnect the power cord and restart but got the same result.

Next I decided to check the PSU. Utilizing a power supply tester, I verified that the 24 pin power, p4 power, and SATA power were all working properly. This leads me to believe that my PSU is functioning properly and the problem might be with the motherboard.

The motherboard exhibits no obvious physical signs of damage (capacitors are not bulging or leaking). It should be noted though that upon first opening this case, there was a substantial amount of dust inside (I mean enough to form mounds at the bottom).

I am a fairly inexperienced technician and am still learning when it comes to diagnosing problems. I believe it could very well be the motherboard but I do not want to overlook something due to my inexperience. Any input on whether or not I am missing something here? Or is my diagnosis of a faulty motherboard sound accurate? Any help is appreciated so thank you in advance!
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TunerMLSystems EngineerCommented:
Significant dust can lead to short circuits. If you are getting correct output during testing then you could most likely elliminate the PSU.
Disconnect all but the power from the motherboard and see what happends, try swapping out the PSU temporarily (from another workstation), to fully ellilminate it.

Try using that PSU with another motherboard (from another workstation).
It could be either, or neither.  Sorry.  If you are going to be testing PCs moving forward, then the correct way to do this is to invest in some motherboard / PSU testing equipment.  

Swapping things in and out isn't the way to go about it. A few hundred bucks  and you can test and diagnose things in minutes with 100% confidence.  

Check out uxd.com.  They have a wide range of products and are probably #1 or #2 in the game.  If your time is worth anything at all, then this is the path you should go down. (Also check ebay to get bargains, of course).
reset RAM modules?
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Scott CSenior Systems EnginerCommented:
I agree with TurnerML...swapping PSUs is the fastest, easiest way to see if it is the power supply.

Not everybody has a few hundred dollars for diagnosic equipment.

I keep a few known working power supplys on hand for cases such as this.

My bet is that it is the power supply as you said the green light was flashing.

Your diagnostic skill seem to be fine to me.  Keep testing parts to find the culprit.
Could also be the switch on the front of the case.
There should be some other outputs from the PS to test such as 5V and 3V.  The pin-outs for the main harness's are easily found online such as these found with Google:
It is not a bad idea to have at least some sort of "load" or power draw connected to the PS while testing.  I would use an old drive.

Given that the power output seems OK, I would suspect the MB.  To avoid getting into the many ways to test the MB directly, I would (only after testing the PS output), try the PS with a "known good" MB (possibly vice versa).  If the MB is bad, the known-good MB will likely work just fine (thus confirming as much).

Of course cleaning the dust from the computer is a GOOD idea.  Also good is reseating the RAM and checking/reseating other connections.

As for purchasing test equipment, PS output testers can be fairly cheap and have good longevity.  A multi-meter may suffice and is more versatile and longer lasting.  As for test equipment for MBs, I used to do that but found that they did not come into play enough before they became obsolete to make them cost effective for a field tech.  Process of elimination and experience was much faster and effective.

An educated process of elimination using component swapping does work as long as you are careful not to fry good equipment with possibly bad equipment, especially a questionable PS.  Components are cheap compared to labor.  If you spend hours in detailed diagnostics of a suspect component that could otherwise be diagnosed with a quick swap, you are wasting your time and your client's money.  Either the client will get wise and you will not see them again or you will end up eating diagnostic time in order to compete with other techs that are more efficient.

 - Tom
According to what you have described I have to tell you that your PSU is failing. Having voltages on the PINs doesn't mean its working properly. Ive seen many HPs with similar issues with green light red light and all fixed by replacing the PSU. Ofcourse there is a possibility but seems to me PSU is the guilty one.
jgretschAuthor Commented:
Thank you TunerML. Even though my power supply tester was showing correct readings, you were right. I have several power supplies around the shop and upon swapping it out with one I knew was working, It fired right up and worked properly. Thank you for your answer and your time!
The tester I have is pretty-much identical to this Coolmax Tester, and it gets HOT if I leave it connected for more than 10 minutes... so I would say it's putting a fairly good load on it, with at least a 10W ceramic resistor.

Many power supplies will tell in their specs what the minimum load needs to be for their regulation circuits to work correctly.
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