The Power_Good signal

Also called Power Good, PowerGood, Power_OK or PWR_OK signal

• A +5 V voltage supplied over a specific wire in the connector that sends pwr from the PS to the mobo.
  
• If the signal is not sent, the mobo will not boot.
  
• If the system won't boot when you press the PWR button, but will finish when you press Ctrl+Alt+Del, this indicates a problem with the Power_Good signal.

1. May not be arriving at the Mobo when expected (right at the beginning of POST) or
2. May not be at the proper voltage.
   
3. Replacing the PS will fix this.