5 Long. That sounds like your CPU. Please reseat, ensure proper alignment. Check socket for bent pins.
Thanks for your reply. I inspected the CPU socket under a magnifying glass for quite a long time and couldn't see any bent or missing pins. I've reseated the CPU about 10 times - it seems to sit down nicely every time and clamp in tightly. It can possibly shift a fraction of a millimetre left-right and up-down before clamping, but I feel like it's within reasonable margins of error.
Is 5 long beeps a standard CPU error code? I know this motherboard has an AMI UEFI BIOS, but I couldn't see any beep codes in the user manual.
My problem is that I bought all my parts from Amazon, so I think I need to figure out which part (motherboard, CPU or RAM?) is at fault, so I can send the right thing back for replacement. Unfortunately, this is the only LGA1151 stuff I have ever bought. Can you offer any tips?