Why won't my machine boot

[xfile102]

My aunt has a Dell Optiplex 390 that won't boot. We swapped out RAM, put in a new power supply and a new Motherboard. Still, when you power the machine on, it runs for 2-3 seconds and turns off. Everytime. We replaced the motherboard, RAM, HDD, and power supply all with brand new parts.. No Joy. What could be the problem?

 

[Metalmario66]

My aunt has a Dell Optiplex 390 that won't boot. We swapped out RAM, put in a new power supply and a new Motherboard. Still, when you power the machine on, it runs for 2-3 seconds and turns off. Everytime. We replaced the motherboard, RAM, HDD, and power supply all with brand new parts.. No Joy. What could be the problem?

Was it having any problems before it started doing this? Did u mess with the bios or settings before this happened?

 

[hang-the-9]

My aunt has a Dell Optiplex 390 that won't boot. We swapped out RAM, put in a new power supply and a new Motherboard. Still, when you power the machine on, it runs for 2-3 seconds and turns off. Everytime. We replaced the motherboard, RAM, HDD, and power supply all with brand new parts.. No Joy. What could be the problem?

A working used model of that computer is $180 on Newegg, why not just replace the system?

Motherboard and power supply are really the only things that can cause it to shut off unless you have a short in the system even after you swapped the parts. Did you try running it on a different outlet?