Is your PSU a modular design, meaning all the cables for motherboards, PCI-E, SATA, ect. are detachable from the PSU itself? If it is, what I mean is attach only the 24-pin motherboard connector to the PSU, leaving the rest unplugged, and attach the other end of the cable to the tester instead of your motherboard. Do you have a link to the tester your using? If your PSU is NOT modular, I would disconnect all the cables from your motherboard, videocard, SATA drives, fans, ect, then again attach only the 24-pin connector to the tester. If your tester can test other cables as well, you can test those individually, though you may still need the 24-pin plugged in regardless. Again, not sure without knowing what your tester is.