When you press the power button this shorts the two header pins on the motherboard which in turn initiates the startup of the PSU. If there is a problem with the motherboard this could result in the PSU shutting down. You can test this by bypassing the motherboard to see if the PSU will start and remain running.
With the computer off, turn off the power switch on the back of the PSU and unplug it from its line voltage.
Open the case and touch the bare metal of the case to discharge any static electricity.
Then remove the 24 pin PSU connector from the motherboard.
Use a paper clip or a small gauge wire to jump between the green wire socket on the connector and any of the black wires socket. Make sure that the connector is not touching any other components inside the case.
Plug the PSU back in and turn the PSU on via the on/off switch.
If the PSU starts and stays running the case fans the problem is with the motherboard or one of its components.
If you wish to test the rail voltages of the PSU with a DMM as previously suggested you need the following information.
There are five different DC rail voltages which are color coded. The Black wires are always negative.
There are only three voltages that can be measured easily without disconnecting the 20/24 pin connector from the motherboard: +12V, +5V, and +3.3V.
The +12V and +5V voltages can be read from a four pin Molex power connector.
Four pin Molex power connector
The same voltages can be taken from a four pin SATA power connector, but in order to read the +3.3V you will need to read this from a five pin SATA power connector as seen below.
Five pin SATA power connector.
To read these voltages you will need to insert the Black (-) probe into any of the black sockets, and insert the Red (+) probe in the different colored voltage sockets. To read the voltages from a SATA power connector it is easiest to insert the probes into the bac k of the connector where the wires enter. Unfortunately the sockets of the modular SATA power connectors are not accessible from the back, so the readings will need to be made from the socket side. Some probes are going to be too large to fit in these sockets, so you may need to insert a piece of wire into the socket of which you want to read the voltage of and place the probe on this for your reading. To reduce the potential of creating a short I would suggest taking the ground potential from another connector so that the two wires will remain physically separated.
Caution: It is very important to make sure that you don't allow the two probes to touch each other when taking the voltage readings. This will cause a short which could damage the PSU or other components.
To get the most accurate readings of the rail voltages it is important that there be a load on the PSU. Running a game or burning a DVD is enough to do this. If you want a program with will put a load on, I would suggest downloading Prime95
and run the Just Stress Test
for this purpose. This program was designed to be used by overclockers to put a full load on the RAM and CPU to determine the stability of their overclocking. Because of this it will put stress on the CPU and RAM which will create higher than normal temperatures. For this reason I would suggest not running this program any longer than is necessary. I would also suggest that an inspection be made of the interior of the case to make sure that there isn’t an accumulation of dust which would impede adequate cooling. Pay special attention to the heat sink and fan assembly on the CPU. If there is a dedicated graphics card with a fan installed on it, look at this fan as well.
Readings should not have variances larger than +/- five percent.
Edited by dc3, 16 May 2016 - 10:46 AM.