I recently bought a dell Optiplex 3040 missing a CPU, RAM, and a HDD. I put in a 120GB SSD, 8GB of DDR3L, and Pentium G4400.

When I tried to turn it on, it failed to POST and beeped 1-3-2. (If I left it plugged in and held the power button to turn it off, It would be off for a few seconds, then turn back on and beep)

This beep code has something to do with bad RAM. I tested the RAM in another PC and it worked. I tried things like using one RAM module, reseating RAM, the password jumper, the CMOS jumper, and removing the PRAM battery. None worked. 

I read in this forum: https://www.dell.com/community/Desktops-General-Read-Only/Dell-Dimension-8400-boot-problem-beeps-1-3-2/td-p/3502116 That people had success when shifting the CPU heatsink.

This did not work for me but I tried reseating the CPU. When I turned it on again, the power light would be white and the fans would spin fast for about a second, then it would do the same beep code. (It didn't output video) I also found the same thing would happen if I moved the RAM module to another slot. (This was pretty consistent)

I tested the PSU with the BIST test button and LED and it passed.

I have no more ideas about what could be wrong, If you have any ideas or something for me to test, please let me know.