PC won't turn on. When i plug in the power supply, the fans move a little but no posting

Mar 5, 2018
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Yesterday i rebuilt my old pc. When i switch the PSU on, the fans move a little and the Ethernet led is on. But when i switch the PSU on without connecting the 4 pin CPU connector, the fans start spinning but no posting. I don't have a case for this machine so i keep it on a carton box. I replaced the CMOS battery and rearanged the RAM but nothing. What could it be? Might the MOBO be dead? (It was working fine 2 years ago) Or just a short? Any ideas on how to solve this problem?