CPU Fan Error

I purchased a PC off Ebay and whenever I start it up, I get a CPU fan error. I have checked all the fans and they are all working. Also, the system is liquid cooled if that has anything to do with it. I don't know much about cooling systems so can someone help me out.

Is the CPU fan plugged into the motherboard or directly into the PSU?

Edit: Somehow missed the liquid cooled part. Yeah, you'll get an error if the fan "socket" on the mobo isn't connected to anything. If you want to stop receiving this message there should be a setting in your BIOS to disable it.

Just plug some boots into it, then you wont get the error.