Checkall internal connections inside your PC, especially power connectors to peripherals. Remove all peripherals and hardware from your computer except your boot drive and graphics card if you Yesuse the power button, you may have to hold it down for at least 5 seconds to force the power off. If this does not work try the other way: force power off, press and hold the power switch for 20 seconds, then wait for ten and restart. Then you proceed to repair the computer: Turningthe PC off and plugging in the 8 pin connectors still makes the PC turn on and off again. The computer will work on integrated graphics so I doubt it's the motherboard. Things I've tried: CMOS reset, reinstall BIOS, buying a brand new power supply. Thebest way to check your PSU is to try to borrow a known good PSU. If you cannot do that, use a DMM (positive lead to colored wires, negative lead to chassis ground or black wires) to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Removeit from the power, hold the power button down for 20 secs to eliminate any stored power in the capacitors. Plug it in you may have a faulty system board. 1 and 3 is the motherboard or power supply. I'd try the PSU diagnostics button and then see about the board. Sounds like the main board. Ifthe required power is not delivered to the motherboard, the computer will fail to start. A common failure point for desktop computers is the power supply box itself. If the system is not vu2G.

computer not turning on power supply ok