A little caveat if trying a PSU tester.  If I remember right [NAME] , 980 MB's have mini 24-pin connection instead of standard 24-pin.  For PSU tester, 24-pin to mini 24-pin adapter would be needed.
I have a Dell Optiplex 980 SFF 12 GB RAM, i3 540, PNY GT 730, Samsung 840 EVO SSD. After a force shut down, error code 3,4 or 3 flashes and the PC won't turn back on for an hour or so. I don't know what causes this but I'm sure somebody out here knows. I don't usually do force shut downs unless it freezes which happens sometimes.
Dell Optiplex 980 SFF error code 34 or 3 after force shut down