My old computer with Intel P4 3.2GHz refuses to power up this morning. The PSU model of my old computer is FSP300-60THN-P (300W). When the main is switched on, I can see a lit green led on the motherboard. Then, I disconnected the 24-pin connector from the motherboard. When the "Power On" (Green) and "GND" (Black) are short-circuited, the PSU's fan is working and all the +12V and +5 are OK. However, the voltage across the "Power On" and the "GND" is found to be less than 4.0V (approximately 3.8V). Then, I plug the 24-pin connector onto the motherboard and measure the voltage across the "Power On" and the "GND" and it's less than 3.0V (approximately 2.6V). So, is my PSU or motherboard faulty? Unfortunately I don't have a spare PSU to verify.