I am Stumped
I have an HP Prodesk 400 G1 SFF at home used for office work. A few months ago, I added a new SSD, an additional 4GB of RAM totaling to 8GB and a new CMOS battery. After those additions, it works perfectly for office work fast and responsive. Now it no longer POST and no longer outputs any video, all it does is turn on and spin the cpu cooler fan and PSU fan at max speed. I first tried a CMOS reset but to no avail. Then I swapped the RAM sticks with a known working one and slotted it in DIMM 1 but that didn't help either. After that I tried the known working RAM in DIMM 3 but that also didn't do anything. I then took everything off, SSD, I/O cables SATA cables and leave the Mobo, PSU, CPU and RAM but that didn't make it past POST either. Later I tried running the PC without any memory and it didn't display anything but it gave me a memory error code which i think was a good sign. I tried swapping the PSU with another known working PSU from another HP Prodesk 400 G1 SFF but that also resulted in no display and the fans running at full power. I've tried reseating the CPU itself and verified if there're any bent pins but all is good. Now I'm thinking it's a corruped BIOS perhaps. Either way, I'm stumped. I hope the good people in this subreddit can help me out troubleshooting this system. Let me know if there's any other information you need me to disclose.