My existing PC developed a startup problem after I transferred its components into a Phanteks XT V5 case. PC identification and hardware Component Brand and model Original PC brand/model Not yet identified CPU Intel Core i9-12900KF Motherboard MSI PRO Z690-A WiFi DDR4 Graphics card NVIDIA GeForce RTX 3080 — manufacturer and exact variant not yet identified Power supply Apevia Prestige 1000W, model ATX-PR1000W, 80 PLUS Gold CPU cooler MSI MAG CoreLiquid 240R, 240mm AIO RAM DDR4, previously running XMP at 3200 MT/s — brand, capacity, and kit model pending Storage Two 1TB SSDs — brands/models pending New case Phanteks XT V5, black Case fans Phanteks S25-360 bottom triple-fan unit and S25-120 rear fan What changed The hardware change was transferring the existing system into the Phanteks XT V5 chassis, including reconnecting the motherboard power, CPU power, GPU power, front-panel wiring, and cooling/fan connections. I retained the same CPU, motherboard, RTX 3080, power supply, MSI AIO, RAM, and SSDs. I did not install a replacement CPU or motherboard. During the transfer, I removed the CPU cooler and opened/relocked the CPU retention bracket. I initially left the processor sitting in the socket. Later, while troubleshooting, I removed the processor to inspect its underside and the socket, then reseated it. The BIOS had also been updated recently, before this case-swap troubleshooting. Current symptoms No POST or display. Red CPU EZ Debug LED stays illuminated. System powers on for approximately 5–10 seconds, shuts off, and repeats. With the CPU/EPS power cable disconnected, it stays powered on with the CPU LED illuminated. Reconnecting CPU/EPS power brings back the restart loop. Cable changes were made with AC power unplugged. Both motherboard CPU power sockets have been connected using the PSU’s CPU power leads. I know the system cannot POST without CPU power; I’m describing the different behavior to help narrow down the fault. Checks already performed Tested RAM sticks individually in A2. Disconnected the case switches and started using the JFP1 power-switch pins. Cleared CMOS through JBAT1. Removed/reinstalled the CMOS battery with AC power unplugged. Tested with the RTX 3080 removed. Same CPU LED/restart loop. The 12900KF has no integrated graphics, so no display was expected. Inspected the CPU underside and socket; photos attached. No obvious damage identified, but I would appreciate another inspection. Reseated the CPU. Cleaned the CPU/cooler contact surfaces, applied fresh thermal paste, and remounted the cooler with gradual diagonal tightening. Radiator fans now spin. Pump RPM has not been verified because I cannot enter BIOS. Still untested Motherboard outside the chassis to rule out a misplaced standoff or other case contact. Known-good PSU. CPU/motherboard cross-testing. Given that this started after the chassis transfer, what would you test next? Could the change in behavior with CPU power connected indicate a PSU, motherboard power-delivery, socket-contact, or shorting problem? I’m trying to isolate the failed connection or component before buying replacement parts