computer won't stay on

Some history please.

Did this just happen "out of the blue"?

Does the light go from green to amber and then stay on amber?

Does the computer have the four LED diagnostic on the rear of the tower?

How do you know it is not the PSU?

Also could be a stuck switch. By default power switches go off after 3 seconds of constant "On" = "Off".

green light — Blinking green in sleep state; solid green for power-on state.

amber light — Blinking amber indicates a problem with the power supply inside the computer. If the system cannot boot and there is a solid amber light, this indicates a problem with the system board (see "Power Lights").

http://support.dell.com/support/edoc....htm#wp1053783

The power button light located on the front of the computer illuminates and blinks or remains solid to indicate different states:

If the power light is green and the computer is not responding, see "Diagnostic Lights."


If the power light is blinking green, the computer is in standby mode. Press a key on the keyboard, move the mouse, or press the power button to resume normal operation.


If the power light is off, the computer is either turned off or is not receiving power.


Reseat the power cable into both the power connector on the back of the computer and the electrical outlet.


If the computer is plugged into a power strip, ensure that the power strip is plugged into an electrical outlet and that the power strip is turned on. Also bypass power protection devices, power strips, and power extension cables to verify that the computer turns on properly.


Ensure that the electrical outlet is working by testing it with another device, such as a lamp.


Ensure that the main power cable and front panel cable are securely connected to the system board.


If the power light is blinking amber, the computer is receiving electrical power, but an internal power problem might exist.


Ensure that the voltage selection switch is set to match the AC power at your location (if applicable).


If the power light is steady amber, a device might be malfunctioning or incorrectly installed.


Remove and then reinstall the memory modules.


Remove and then reinstall any cards.


Remove and then reinstall the graphics card, if applicable.


Ensure that the processor power cable is securely connected to the system board.


Eliminate interference. Some possible causes of interference are:


Power, keyboard, and mouse extension cables


Too many devices on a power strip


Multiple power strips connected to the same electrical outlet

http://support.dell.com/support/edoc....htm#wp1054146

To help you troubleshoot a problem, your computer has four lights labeled "1," "2," "3," and "4" on the front panel (see "Front View of the Computer"). When the computer starts normally, the lights flash. After the computer starts, all four lights display solid green. If the computer malfunctions, the color and sequence of the lights identify the problem.

Diagnostic Lights