Question Dell optiplex 990 turns on automatically and no display

Mar 24, 2020
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I have Dell optiplex 990 it was working fine.I clean it and after assembling all parts I plug the power cable in psu and my PC starts automatically and power button is flashing Amber light also 1 & 3 diagnostic lights are flashing and psu rear/back light is flashing
 
found this troubleshooting guide
https://www.dell.com/support/article/de-de/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en

A possible system board, power supply, or peripheral failure has occurred.​
  • Power off computer, leaving the computer plugged in. Press and hold the power supply test button on the rear of the power supply unit. If the LED next to the switch illuminates, the problem may be with your system board.
  • If the LED next to the switch does not illuminate, disconnect all internal and external peripherals, and press and hold the power supply test button. If it illuminates, there could be a problem with a peripheral.
  • If the LED still does not illuminate, remove the PSU connections from the system board, then press and hold the power supply button. If it illuminates, there could be a problem with the system board.
  • If the LED still does not illuminate, the problem is probably with the power supply.


EDIT
Did you connect the 4 pin power connection P2 to the motherboard as well?
 

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