In the summer this year, I rebuilt one of my machines. It had a 2.4 GHz Core 2 processor. I rebuilt with a 4GHz i7, huge heatsink, on an ASUS Sabertooth, (their spelling), Z170 motherboard and 16GB RAM and a Corsair CX750 PSU. Case, SSD and other stuff remained. I was REALLY pleased with it, so much so, that I advanced my timescales and decided to rebuild my other machine this year as well. Exactly the same parts. It didn't start. I've just compared the spaghetti of twisted pairs between the two machines figuring a goof there, easy to do, but it seems okay. Now, of course, I can start swapping pieces between machines to see if I've got a dodgy part, but that is a real pain. Anything simple I can look at or try before going there?