X61s weird fan error

[crazyfrog]

My friend bought a second hand working X61s and and asked me to help verify its spec.

I removed the keyboard and upper case (palmrest) to take a look inside of it. This X61s comes with wireless WAN, so it has a small fan near the speaker. But the small fan was not connected. So I thought someone must forget to do that and I put the small fan plug in the socket on the system board. But to my surprise, when I turned it on the laptop reported fan error and shuted itself down immediately. Then I unplugged the fan, it booted into system with no error. Then again I thought this fan may be dead. I happened to have another working small fan. So I plugged my fan in the socket and turned it on. It reported fan error and shuted down too!

I know every X61s with wireless WWAN should come with this secondary small fan; otherwise, it will report fan error during boot, although one can bypass this warning by pressing Esc. But I don't understand why this X61s does not like the small fan at all. BTW, I noticed both small fans did spin for a couple of seconds along with the main fan when I turned the X61s on. So it is not a connection problem. I've also upgraded its BIOS to the latest version before the test.

Any hints? Thanks in advance.