I had this same error code. C5 is not a defined error, but after a bit of googling, I learned that it likely is a problem with memory. In my case, I had 2x16GB memory sticks, and one of them was bad and had to be replaced.
Here are some things you can try to find the root cause.
1. **Ensure RAM is compatible**: First, make sure that the RAM you bought is compatible with the motherboard. Have a look here:
https://www.gigabyte.com/Motherboar.... You can filter by different criteria, or you can search directly for the module part number. For example, I have
Kingston D5 32GB 6800-36 Renegade wh K2 KFY XMP (2 x 16GB, 6800 MHz, DDR5 RAM, DIMM) and module part number is listed on the Galaxus product page as "Manufacturer number: KF568C36RWK2-32". KF568C36RWK2-32 you can just search directly on the memory support page. If your RAM is not listed, get some that's supported.
2. **Install RAM in the right slots***: There are 4 channels/slots: A1, A2, B1, B2, ordered from left to right. For a single stick, use slot A2 only. For 2 sticks, use A2 and B2. Etc. See Page 15 of
https://download.gigabyte.com/FileList....
3. **Isolate a potentially bad memory stick**: If you have multiple sticks, try installing them one at a time in the A2 slot (the second from the left) and see if you can get it to boot. This is how I found out I had a bad stick.
Good luck!
