If you haven't ripped off everything already... : ) With all external peripherals disconnected, with PC off, press the test button to see if it gives you a green light.  If it does, that doesn't necessarily mean good PSU, but means internal peripherals are good.  If not, disconnect internal peripherals one by one, testing for green light in between.  For example, once it's down to the MB, and then you get a green light once it's disconnected, the MB is the problem.  Just another way to verify it.  If all internal peripherals give you the green light, then it could be that the PSU is a putts. Dell BIST Test 
hi all  i install an os on a OptiPlex 9020 sff  after a while the power button blink amber the hdd light blink and i hear a beep  what could it beep the issue ?
Optiplex 9020 Sff beep code and light issue