I recently swapped out my brother's motherboard to upgrade to one that supports his new SSD. We were using an ASRock B450 Pro4 R2.0, but I had a Gigabyte B450 Aorus Elite that worked perfectly with the SSD when I tested it. Now that everything is plugged back in, his PC refuses to boot and the red CPU light is continuously on. Both our systems have Ryzen 5 5600 CPUs, and I've even tried using his old Ryzen 3 1200 along with my RAM modules, but no luck. I did attempt to update the BIOS with the Ryzen 3, but it still wouldn't boot. Has anyone experienced this and have suggestions on how to fix this?
3 Answers
It sounds like your motherboard might not be recognizing the CPU properly. Have you checked the CPU power connections? Sometimes the 8-pin connector can come loose, which causes these issues. Also, ensure that there aren't any bent pins on the CPU itself. If those look fine, consider reseating the CPU and checking if it moves the red light situation at all.
I had a similar issue once, and it was related to the GPU. You mentioned that the red light doesn't come on when the GPU isn't plugged in, right? Try booting the system without the GPU connected to see if it will post. If it does, the GPU might be faulty or not seated correctly.
Another thing to check is the motherboard’s RAM slots. Try booting with just one stick of RAM and switch between slots. Sometimes a faulty slot can cause boot issues. If it still doesn't boot, the motherboard itself might be defective.

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