I have purchase Dell Optiplex 780 that came with 8GB (4x2GB) DDR3 memory. I wanted to upgrade it so I bought 4GB (4x1GB) RAM and tried to install it today. The Dell diagnostic lights lit up 3 and 4 which means that the memory is detected, but at fault. Now I doubt that new memory module are defective. Here's what I did to troubleshoot:

I took the original memory modules out and installed the new one. There are 4 DIMMs and I tried to put it out in all 4 DIMM sockets one by one but my PC won't be start and shows Dell diagnostic lights lit up 3 and 4. then I put my original RAMS but still same issue occured I also tried both Original RAMs one by one in all slots but same issue :(

I also tried with no memory installed but still Dell diagnostic lights lit up 3 and 4 and no beep heard.

Can anyone helps me out what the actual problem is ???